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MYSTERIES OF TIME 



AND SPACE 



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BY r 

RICHARD A.' PROCTOR 



AUTHOR OF 'PLEASANT WAYS IN SCIENCE* ' THE EXFANSE CF HEAVEN 1 
'FLOWERS OF THE SKY* ETC. 



V 



^ 







One music as before, but vaster' 

Tennyson 



WITH TWENTY-FOUR ILLUSTRATIONS 




NEW YORK 
R. WORTHINGTON, 770 BROADWAY 

1883 






Hy Transfer 
\ \308 






PREFACE. 

The essays in the present volume are for the most 
part among those in which I have endeavoured to 
present the tendency of science in our time to show 
the domain of law as to all intents and purposes 
infinite. The mysteries of time and space here dealt 
with were no mysteries to those men of old times who 
supposed that at a short distance from this earth lies 
the region which they regarded as the heaven of 
heavens, and that a short- distance in time from the 
present takes us to the beginning of all things in the 
past, to the end of all things in the future. Starting 
with such conceptions- — the positivism of early thought 
— men found more and more of mystery as they 
learned more and more of actual facts, until now that 
which of old was confidently explained is found to 
be utterly inexplicable, save as a part — an infinitely 
minute part — of the mysterious Infinity and Eternity 
we call The Universe. 

RICHARD A. PROCTOR. 

London : March 1883. 






CONTENTS. 



PAGE 

Newton and Darwin i 

The Vistas of the Past 14 

The Birth of the Moon 30 

Birth and Death of Worlds 55 

The Sun as a Perpetual Machine 80 

The Sun's Corona 94 

The Sun's Long Streamers no 

Meteoric Astronomy 124 

Comets 153 

Cometic Mysteries 184 

Dangers from Comets 204 

The World's End 229 

The Menacing Comet 243 

Jupiter's Satellites 259 

Terrestrial Magnetism 272 

The Star-depths 287 

Transits of Venus . . 307 

Star-clouds and Star-mist 334 

Herbert Spencer's Philosophy 357 

A Survey of the Northern Heavens . . . . 378 

Star unto Star 4 02 



MYSTERIES OF TIME 
I AND SPACE. 

NEWTON AND DARWIN. 

I It is singular that the theory which — of all those advanced 
since Newton established the law of gravitation — has given 
to thoughtful minds the grandest conceptions of Nature and 
the laws of Nature, should have been, of all theories perhaps 
ever suggested by man, the most thoroughly misunderstood. 
There can be no doubt that many who recognise the real 
I significance of the theory of natural development, who know 
, that its influence is by no means limited to biological evo- 
lution, but has been felt in the far wider — the infinitely 
wide — field of cosmical evolution, have been pained by the 
thought that with the widening of the domain of develop- 
ment, the belief in a power working in and through all 
things seems to be set on one side in the name of universal 
evolution. It is this thought — this fear it may be called 
f ' perhaps — which I propose to consider here. I shall en- 
I ; deavour to show that those who are perplexed by such 
| doubts overlook the parallelism which exists between three 
I lines along which men's thoughts have been carried an ever- 
| increasing distance, until it has become obvious that two of 
I them at least must be infinite — that the fear expressed by 
1 those who see with anxiety the progress of evolutionary 
j doctrine implies a hope that one of these lines may be 

B 



2 MYSTERIES OF TIME AND SPACE. 

finite while the others are essentially infinite, and are ac- 
cepted as such without fear or trouble. 

It was a new thought in the time of Copernicus, that 
men had hitherto underrated the extent of the universe, and 
had overrated the importance of our earth. The globe, which < 
had seemed the one fixed orb for whose benefit the heavenly I 
bodies had all been made, was found to be but one member 
of a family of orbs circling round a globe much larger than 



any of them. Thus the earth lost at once her central posi- 
tion, her quality as the world (the sole abode of life), her ) 
fixity, her importance in respect of the supposed superiority i 
of her dimensions. When Newton had finally established 
the Copernican theory, 1 the relative insignificance of the 
earth was demonstrated. The teachings of the telescope 
showed in turn the insignificance of the solar system. With 
every increase of light-gathering power the universe of stars 
grew larger and larger, even when as yet no scale had been 
obtained whereby to determine the distance separating star 
from star. With every improvement in the defining qualities 
and the measuring power of telescopes, the universe of stars 
grew larger and larger, independently of mere increase in 
number of stars ; for though for a long time no measure- i 
ment of star-distances could be effected, each failure with / 
improved means to measure the distances of even the \ 
nearest stars showed that the scale of the stellar universe 
was larger than had before been imagined. 

Larger and larger grew the universe, then, as men turned 
more and more powerful, more and more exact instrument 
to the survey of the heavens. When at length the distance 
of the nearest star was measured, and found to be more than 
twenty millions of millions of miles (more than three years' 

1 It is worthy of notice that that theory could not be regarded as 
demonstrated until the law of attraction had been established. This 
law carries with it the disproof of the cycles and epicycles of the Ptole- 
maic system, because under the law of gravity bodies cannot move in 
such curves. Before the law was established, it was more probable 
that the planets all moved in simple curves, but not certain. 



NEWTON AND DARWIN. 3 

light-journey, though in each second light travels a distance 
exceeding nearly eight times the entire circuit of the earth), 
the number of stars was already known to exceed twenty 
millions. But more powerful telescopes have been made 
since. With every increase of telescopic power more stars 
come into view. With such a telescope as the great reflector 
of Parsonstown, at least a hundred millions of stars could be 
seen if every part of the stellar sphere could be scrutinised 
with that mighty telescopic eye. 

But what, after all, is this? Now that we know how 
minute a creature man is, how insignificant his largest works 
compared with the globe on which he lives, how this globe 
is but a point in the solar system, the solar system lost 
among countless millions of other suns with their attendant 
planets, how preposterous appears the thought that any in- 
strument man can fashion can penetrate the real profundities 
of the universe ! Seeing, as we do now, how utterly men's 
ideas of what the stars are fell short of the truth, and how 
more inadequate still were their conceptions of the real 
number of the stars when they trusted only to the natural 
eye, we should very ill have learned the lesson their errors 
teach us, if we in turn fell into the mistake of supposing 
that the telescopic eye can reveal more to us than the 
merest corner of the universe. Even of the universe of 
stars — that is, o the system of suns whereof our sun is a 
member — this may be said. But how unlikely, how in- 
credible, indeed, is it that there is but one system of suns, 
but one galaxy? The star-clouds may not be outlying 
galaxies, as the Herschels supposed. It seems clear that 
they are but parts of our own galaxy, whose grandeur and 
complexity are far greater than had been supposed. But 
who can doubt that beyond the limits of our own galaxy, 
beyond spaces bearing probably something like the same 
proportion to the size of the galaxy that the interplanetary 
spaces bear to the size of our earth, come other galaxies, 
some like, some unlike, our own, some as large, some 
smaller, but many doubtless far larger than the glorious 



MYSTERIES OF TIME AND SPACE. 



system of suns which appears infinite to our conceptions? 
' As thus we tilt ' — in imagination — ' over an abysmal world, 
a mighty cry arises that systems more mysterious, worlds 
more billowy, other heights, other depths are coming, are 
nearing, are at hand.' Who can wonder if from these awful 
depths men have turned in weariness of soul, nay, almost in j 
affright, as when the Alpine traveller, peering over some 
fog- enshrouded precipice, sees down, as the mist rolls past, 
to deeper and deeper abysses, until he is compelled to turn 
from the contemplation of the ever-growing depth ? It is \ 
not simply the vast in which men have learned to believe, 
not mere immensity, but the mystery of absolute infinity. 
On all sides our island home is surrounded by a shoreless 
sea of space. So. great has been the oppression of this 
mystery of infinity that men like Helmholtz, Clifford, and 
others have attempted, by rejecting the elementary con- 
ceptions of space, to show that there may be limits to space 
— not merely limits to occupied space, but limits to space 
itself— as though by closing his eyes, the traveller, oppressed 
by the vastness of the plain surface over which he voyaged, 
should endeavour to convince his mind that the end of his [ 
journey was close by him. 

' Practically infinite,' as Huxley has expressed it, or \ 
absolutely infinite, space is (to all intents and purposes) j 
infinite for us. But space and time are too intimately asso- / 
ciated for us to imagine that space can be infinite and time j 
finite ; or that if occupied space grows, even under our 
survey, until we recognise that it is as infinite as space itself, - 
time occupied by the occurrence of events (of whatever sort) 
can be otherwise than infinite too. j 

If we could reasonably doubt this we should yet find 
evidence as clear in this direction as with reference to space 
itself, though not so obvious to the senses. Everyone can j 
understand the evidence of vast size presented by the uni- 
verse as science is able to survey it ; and everyone can see 
how the constant growth of the known universe points to the 
real universe as to all intents and purposes infinite. But not 



I 



NEWTON AND DARWIN. 5 

everyone can understand the evidence of the antiquity of 
the universe, or the certain promise which its features afford 
of a duration in the future which must be, like the duration 
of the universe in the past, practically infinite. But even to 
those who cannot see the force of the evidence on these 
points, it is obvious, so soon as the idea has once been 
presented — just as obvious as is the idea of infinite absolute 
space — that time itself, occupied by events or not so (if this 
could be imagined), must be absolutely infinite. The occur- 
rence of events might perhaps be spoken of (not conceived 
very readily) as having an absolute beginning and proceeding 
onwards to an absolute end, this island of occupied time 
being lost in a shoreless ocean of void time ; but none can 
reasonably speak even of a beginning or an ending of abso- 
lute time, far less conceive either thought. 

Space, then, and time present themselves to our concep- 
tions, and with the progress of research may be said to 
present themselves to our observation, as practically infinite. 
The earth, which has been displaced from her imagined 
central position in space, has been displaced equally from 
her imagined central position in time. The ocean of time 
which had been supposed bounded on one side by the 
beginning of this earth's history and on the other by the 
close of the earth's career, is seen to bear somewhat the 
same relation to the earth's duration that the Pacific Ocean 
bears to the tiniest islet of the least important Polynesian 
, group. 

Now in the days when the earth was thought to be 
central and all-important in space, central also and all- 
important in regard to time, a little knowledge — as limited 
and as imperfect — was possessed by men respecting the 
action of natural laws. They knew for example that animals, 
including man, pass through certain stages of development. 
They saw that the trees of the forest spring from seeds. 
They could trace further the growth and development of 
families of animals, the spread of vegetation over countries 
and continents ; the formation, on the one hand, of tribes, 



6 MYSTERIES OF TIME AND SPACE. 

nations, races, and species ; on the other, of the various 
forms of vegetable development. But such knowledge, and 
all the ideas associated with such knowledge, were limited 
within the range of space and time over which alone in 
those days men were able to extend their survey. In fine, 
men recognised processes of development taking place upon 
the earth, and during her continuance as an inhabited world ; 
they did not look outside either the region of space or the 
period of time which they had learned to regard as if they 
were in reality all space and all time. 

In passing I may note that hitherto I have not heard 
that in the good old days — when the earth was the world 
and her life (very much under-estimated) all time — men who 
studied processes of development or evolution such as are 
plain and obvious to all were regarded as necessarily re- 
jecting the belief in some power at the back of observed 
phenomena. On the contrary, so far as we can judge of 
the ideas of those days by what men said, it would seem to 
have been regarded as a wholesome thought, that under 
the operation of natural laws trees and animals, races andl \ 
forests, grow from feeble beginnings till they fulfil all the \ 
functions of their several kinds. The more carefully such j 
processes of development were considered, the more per- ( 
fectly the laws of Nature seemed fitted to work out their I 
seeming purpose, so much the more confidently did men 
regard those processes and laws as implying some plan or / 
purpose j though also, it must be admitted, the nature of / 
such plan or purpose seemed to the wiser sort the more [ l 
inscrutable the more closely its workings were studied. | 
* Canst thou by searching find out God ? ' said one, who so 
far spoke truth, though he drew the wrong lesson from it ; 
' canst thou find out the Almighty unto perfection ? It is 
as high as heaven ; what canst thou do ? deeper than hell ; 
what canst thou know?' Another, who took a wiser view of 
Nature, yet in this spoke the same doctrine : ' Touching 
the Almighty, we cannot find Him out.' 

In our day, with the extension of men's recognition of 



NEWTON AND DARWIN. 7 

the vastness of space and time, there has come a widening 
also of their conceptions respecting the extent of the domain 
of natural law as well in time as in space. 

And in the first place I would ask whether it is not 
naturally to be expected that this growth in our ideas 
respecting evolution should have followed ( if it did not 
accompany) the growth of our conceptions of the extent 
and duration of the domain of evolution? It it had so 
chanced that neither research nor observation had availed 
to extend our recognition of the operation of natural laws — 
after Copernicus, Kepler, and Newton had established the 
true theory of the solar system — might not analogy alone 
have sufficed to convince men that the larger and longer- 
lasting universe shown them by science was governed by 
wider and more permanent laws than they had hitherto 
recognised ? 

But the Copernican theory had not been established 
without the demonstration of a law so general and far- 
reaching that when it had once been established no new 
recognition of- law could be reasonably regarded as startling 
or unexpected. Newton had proved that the quality of 
gravity pertains to every particle of matter in all places and 
in every condition, and that it extends according to definite 
law to an infinite distance. At least, he had proved these 
properties so far as they can be proved. Every possible test 
had shown that the particles of solid, liquid, and vaporous 
matter equally possess (according to their mass) the quality 
of gravity. Every possible test had shown that not the 
external particles of suns and planets, or these in greater 
degree, but every particle, to the very centre of the largest 
and most massive globe, possesses in the same degree 
(according to its mass) this mysterious, all-pervading power. 
And lastly, every possible test applied to the movements of 
the heavenly bodies had shown that the force of gravity 
exerted thus by each particle diminishes as the square of 
the distance increases, but suffers no further diminution : so 
that the tiniest particle in the sun exerts, at least throughout 



'■' 



8 MYSTERIES OF TIME AND SPACE. 

the domain of the solar system, even to the orbit of Neptune, 
the force due to its mass and to the distance of any other 
particle on which its influence is exerted. In this inquiry 
the vast mass of the sun stands us in good stead. Were 
we only able to consider the attraction exerted by a single 
particle, or by a small mass at great distances, the smallness 
of the resulting attraction would foil any attempt to measure 
its amount with precision. But we can consider the total 
energy of the solar mass, exceeding 350,000 times the mass I 
of the earth, at the distance of Neptune ; in other words, we \ 
can examine the combined attractive force of a gathering of I 
many millions of millions of particles, and having measured ! 
that, we can divide it in accordance with the known relative ! 
mass of the sun, and so ascertain whether each particle of 
the sun does its due work at the distance of Neptune. 
When we thus learn that there is not the slightest trace, 
even over that enormous range, of any diminution of energy 
beyond that belonging to the law of gravity as determined 
for a small distance (such as the moon's), we are justified in ! 
assuming that at a distance twice, thrice, many times as ; 
great as Neptune's the law of gravity holds unchanged. We j 
have then a law whose action is to all intents and purposes 1 
universal ; it operates in every particle of the universe, and 
it extends from particle to particle throughout the whole 
extent of the universe. Of a law such as this, if of any law 
at all, it might have been said that it seems to negative the I 
action of a special Ruler. It was said of late respecting ' 
the general doctrine of development, that it sets the Almighty ' 
on one side in the name of universal evolution ; with at e 
least as much force it might have been said of the doctrine J 
of attraction, that it sets the Almighty on one side in the | 
name of universal gravitation. I 

We know indeed that such an objection was urged \ 
against Newton's d octrines in Newton's day and for many 
years after. Very probably if the theory of gravitation had 
not been established to demonstration by Newton and such 
followers as Laplace, Lagrange, and others, we might hear 



NEWTON AND DARWIN. 9 

the objection even now (we hear it still among the ignorant, 
but of course it has entirely died out save with them). When 
the theory of universal gravitation became thoroughly es- 
tablished, it was found to be in perfect accordance with the 
idea of a universal lawgiver. Men presently began to won- 
der, indeed, how it could ever have been supposed that the 
laws of the universe must of necessity be limited in their 
range of action whether in space or in time. 

Yet when the Newton of our own time advanced a theory 
which bears to biology (so far as is possible in matters so 
unlike) the same relation that the law of gravity bears to 
astronomy, a theory bringing animal and vegetable life under 
the domain of laws practically universal, an unreasoning fear 
possessed many lest this natural sequel of our growing 
knowledge of the universe should alter men's conceptions of 
the government of the universe. In space the universe was 
seen to be infinite, and in duration infinite ; a law infinitely 
wide in its operation had been found to govern all move- 
ments within the universe, yet the recognition of a new law, 
also indefinitely wide in its operation, instead of being re- 
garded as natural and appropriate, was looked upon with 
disfavour and disapproval. 

Note that we use the word indefinite, not infinite, in 

speaking of the operation of the law of biological evolution. 

The biologist cannot test the operation of this law so widely 

i as the astronomer can test the operation of the law of 

1 gravity, for the simple reason that the biological law relates 

I chiefly to time, while the astronomical law relates chiefly to 

I space, and we can look with ever-increasing range of vision 

into depths of space which are practically infinite, while we 

cannot look with equal confidence into remote depths of 

past or future time. For the same reason that men even 

to this day accept more confidently the enlarged ideas of 

science with regard to space than the extended ideas with 

regard to time, which logically should be accepted with 

equal readiness, the theory of evolution must ever remain 

incomplete as compared with the theory of universal attrac- 



io MYSTERIES OF TIME AND SPACE. 

tion. No one could urge with much effect, in these days, 
that perhaps beyond the range of the telescope the law of 
gravity which within that range (and far beyond the limits 
of the solar system l ) we see in operation, may be replaced 
by some other law entirely different in its mode of action. 
But the opponent of the doctrine of biological evolution 
may, without much fear of effective reply, express the belief 
that before some definite epoch in the past, not evolution, j 
but some other law or process, was at work in the fashioning 
of the various forms of animal and vegetable life. In deal- 
ing with space no one can reasonably say, that in whatever j 
direction one may suppose a line extended, a limit must at 
length be reached beyond which we cannot, even in imagi- 
nation, extend our survey. But in dealing with time it is 
not considered unreasonable, but, on the contrary, eminently 
reasonable, to say that far back as we may please to carry 
the process of evolution we must at length come to a be- 
ginning, before which there was not only no evolution of life s 
but no life to pass through processes of evolution. 

Here, indeed, science assents in some degree to the 
objectors, if science may not be said to have given birth to 
the objection. Science has shown that with suitable care 
to remove or destroy all germs of life from a given space, 
no life will appear within that space — in other words, that, 
so far as scientific observation extends, the generation of 
life is never spontaneous. Equally science might assert / 
that, so far as scientific observation extends, the generation | 
of a system of orbs like the solar system does not occur \ 
spontaneously under any suitable test conditions. If a \ 
smile be excited by the thought of the vast difference of j 
scale between any test conditions and the conditions 
under which our own solar system may have come into j 
being, let it be noted that there must be a kindred differ- i 
ence between any experiments as to the possibility of spon- i 

1 Binary, triple, and multiple star-systems tell us of the operation 
of gravity in the star-depths ; and so do the movements of stars in 
space, though not so obviously. 



NEWTON AND DARWIN. n 

taneous generation and the only conditions under which 
we can imagine spontaneous generation to have occurred. 
There is some difference, we submit, between a small 
flask with a few ounces of hay infusion, to which no air 
has been admitted, which has not been submitted to a 
number of life-destroying processes, and a young planet 
teeming with material vitality, still hot with its primeval 
fires, still palpitating from the throes which (during countless 
ages) had preceded and accompanied its birth. No experi- 
ment or observation man has ever made or can ever make, 
can suffice to show that the spontaneous generation of living 
forms then was either possible or impossible. But men may 
continue, if it gives them any comfort, to believe that just 
then the uniform action of law was interrupted, that just 
at that stage the mechanism of the universe was found to 
be imperfect. 

But while in this sense and to this degree the law ot 
biological evolution differs from the law of universal attrac- 
tion, the work of Darwin must yet be regarded as akin to 
that of Newton, in that it extends indefinitely our concep- 
tions of the range of natural laws. As Newton showed men 
all the millions of families of worlds throughout the universe 
moving in accordance with the law of attraction, so Darwin 
has shown us all the myriads of races which have inhabited 
the earth brought into due relation to their surroundings by 
the operation of the law of evolution. And as the law of 
gravity was but a wider law, including such laws as Coper- 
nicus and Kepler had recognised, which in turn severally 
included many minor laws, so it should be noticed that the 
law of biological evolution includes all those minor laws of 
development which men had recognised for ages without 
entertaining the unreasonable thought that such laws neces- 
sarily implied the non-existence of a lawgiver. 

To those alike who are pained and to those who rejoice 
at what they regard as the irreligious tendency of the doc- 
trine of biological evolution, the same answer maybe made: 
it is only when we try to create arbitrary limits of space or 



12 MYSTERIES OF TIME AND SPACE. 

of time, and to set these as bounds to the operation of the' 
laws of Nature, that any such tendency can be imagined. 
Those who have admitted the growth of a tree, a forest, or 
a flora, of an animal, a race, or a fauna, according to natural 
laws, have to acknowledge nothing new in kind, however 
different it may be in degree, in admitting that there is de- 
velopment on the larger scale as well as on the smaller, not 
even though they should have to admit that such develop- 
ment takes place throughout all space and all time. The 
difficulty in dealing with one thought is not greater than 
that which oppresses us in considering the other ; both 
difficulties are overwhelming, both infinite. If we could 
evade the conception of the infinite in space or in time, we 
might be content to imagine limits to the operation of law. 
But we can neither evade the conception nor grasp it. As 
Pasteur has well said, quite recently — ' When the question 
is asked, " What is there beyond the starry vault ? " it is use- 
less to answer, " Beyond lies unlimited space." When we ask 
what lies beyond the far-off time when what we see around 
us began to be, and what lies beyond the remote future when 
it will cease to exist, of what use the answer, " Beyond lie 
eternities of past and coming time " ? Nobody understands 
these words. He who proclaims the existence of an Infinite 
— and nobody can evade it — asserts more of the supernatural 
in that affirmation than exists in all the miracles of all the 
religions ; for the notion of the Infinite has the twofold, 
character of being irresistible and incomprehensible. When 
this notion seizes on the mind, there is nothing left but to 
bend the knee. In that anxious moment all the springs of 
intellectual life threaten to snap, and one feels near being 
seized by the sublime madness of Pascal. Everywhere I see 
the inevitable expression of the Infinite in the world. By 
it the supernatural is seen in the depths of every heart/ 

It is as thus viewed that the laws of development brought 
before us during the last quarter of a century — not as novel- 
ties, for in conception they are of vast antiquity, but new in ' 
the sense that now for the first time they are presented as 



NEWTON AND DARWIN 13 

proven — are so solemn and impressive when rightly under- 
stood. As the discoveries of astronomy were first steps 
towards infinite space, steps carrying us far enough upon 
the road to show that of necessity it must be infinite, as the 
study of the movements of the heavenly bodies tells us 
unmistakably of infinite time, so the recognition of develop- 
ment tells us that, as we might have anticipated, the domain 
of law is limitless alike in space and in time. With the 
angel in Richter's Dream, Science, in the doctrine of Ever- 
lasting Evolution, proclaims the solemn truth, — ' End is 
there none to the universe of God ; lo, also, there is no 
Beginning.' 



14 MYSTERIES OF TIME AND SPACE. 



THE VISTAS OF THE PAST 1 

Many of those who follow with interest the teaching of 
science, but have not leisure to study carefully the methods 
and principles on which those teachings depend, are inquir- 
ing what new views are these according to which the moon 
was born of the earth many millions of years ago, and has 
been retreating ever since from the parent orb ; how those 
views are related to the nebular hypothesis of Laplace ; and 
what bearing they may have on astronomical and geological 
estimates of past eras in the earth's history. An eloquent 
lecture by the Astronomer Royal for Ireland has done much 
to increase the interest with which these questions are 
viewed ; indeed, it may be doubted whether many who are 
now inquiring about these matters had heard of them at all 
before Dr. Ball brought them before the attention of the 
audiences to whom his lecture has been addressed. 

I propose to sketch the ideas resulting from the re- 
searches of Mr. George Darwin, noting how they are related 
to former views respecting the development of the solar 
system, and how they bear on certain other astronomical 
and geological theories. At the outset I may remark that t 
cannot altogether agree with the opinions expressed by Drr 
Ball, and to some degree by Mr. Darwin, respecting th 
manner of the moon's birth ; but as to the general theory t 

I 
\ 

1 This essay should be read in connection with that which follows. 
It will be found that they are in a sense supplemental to each other, por~ 
tions which are fully treated in one being summarised in the other, andl 

vue versd. I 






THE VISTAS OT THE PAST 15 

which Mr. Darwin's researches have led there seems very- 
little room for doubt or question. 

In carrying back our thoughts to the past of the earth, 
our most trustworthy guide (though we must be careful in 
following even this guide) is evidence found in the study of 
processes actually taking place at the present time. For 
instance, we find that the earth is slowly cooling. We can, 
therefore, go back to a time when she was much hotter than 
she is at present ; and though we may not be able to assume 
confidently that her temperature was ever so great as to 
cause every particle of her substance to be vaporised, we may 
infer even that, if other features actually existent seem readily 
explicable on such an assumption. Again, we find that the 
earth gathers in every year hundreds of millions of meteoric 
masses of greater or less weight, down to bodies weighing 
only a few grains ; and we know from the orbits followed by 
the greater number of these that they belong to systems 
travelling around the sun on paths of such a nature as to 
forbid us to believe that they were originally expelled from 
the earth. Seeing, then, that the earth is gathering in 
materials from without, though now at a very slow rate, and 
seeing further that this process is of necessity one which 
takes place more and more slowly as time proceeds, we are 
justified in looking back to a time when it progressed far 
more quickly than at present, in considering that over the 
whole intervening period — many millions of years — it has 
been at work, and finally in inferring that no unimportant part 
of the earth's present mass has been derived in this way from 
meteoric aggregation. 

Now, among other processes of change that are taking 
place in the earth and her dependent or associate orb, the 
I moon, are two others, discovered in comparatively recent 
i times, though not quite so recently as some might infer from 
j Dr. Ball's account. About a quarter of a century ago Pro- 
Ifessor^Adams, co-discoverer with Leverrier of the distant 
I Neptune, announced that he had discovered an error in 
Laplace's discussion of the so-called acceleration of the 



1 6 MYSTERIES OF TIME AND SPACE. 

moon, and that when this error was corrected the accelera- 
tion could not be entirely accounted for by the theory of 
gravitation. It was presently shown by the eminent astro- 
nomer Delaunay (not to be confounded for a moment with 
the Delaunay who has recently insisted on the inferiority of 
the weaker sex) that this unexplained part of the accelera- 
tion of the moon may be explained on the assumption that 
it is not the moon which is gaining, but the earth which is 
losing time ; in other words, that the great terrestrial clock, 
the rotating earth, by which we measure time, is not going 
at a uniform rate, but is gradually losing its rotation spin. 
Laplace's assertion that the earth's rate of rotation, so far as 
astronomy can measure, is appreciably constant, was based 
on his investigation of the moon's so-called acceleration. 
Supposing that no part of this change remained unexplained, 
when solar and planetary perturbations of the moon were 
taken into account, he naturally inferred that the great ter- 
restrial timepiece is keeping most perfect time. Finding, 
on the contrary, that a part of the acceleration does remain 
unexplained, we are justified in assuming, as at least a pos- 
sible interpretation of the excess of acceleration, that our 
chief timepiece is losing time. Delaunay pointed to the 
tides as a probable and sufficient cause of this change — the 
great tidal wave carried, not bodily, but still swayingly, 
against the direction of rotation, checking the earth's rotation 
spin slowly but 'exceeding surely.' 

Next, it was shown that, accompanying this change, there 
must be a gradual loss of lunar motion, accompanied by a 
gradual recession of the moon. 1 

In the essay which follows, on The Birth of the Moon^ 
I describe more at length these two processes of change. 

1 This may seem inconsistent with what we said above about the 
lunar acceleration which astronomers have endeavoured to explain. 
But this acceleration is one of the temporary changes which the moon's/ 
motion undergoes. It alternates with a similarly temporary retarda-f 
tion, in periods of great length indeed, but not to be compared with 
the enormous time -intervals which we are considering. 



THE VISTAS OF THE PAST 17 

Here, for the present, let it suffice to note that astronomy 
recognises them as taking place, and that they therefore are 
among the processes which we may carry back in imagina- 
tion to a very remote past, in order that we may recognise 
what probably was the initial condition — at any rate, a very 
early condition — of the orbs in which they are taking place. 

Of course it is an obvious thought that if the moon is 
thus receding now, and has been receding in the past, she 
will one day part company with the earth altogether, and 
that she was at one time quite close to the earth, and even 
a part of the earth's mass. Considering, also, the change in 
the earth's rotation period, and carrying our thoughts as far 
back into the vistas of the past for this change as for the 
other, we see a time when the earth was rotating so fast 
that its equatorial parts were barely restrained by gravity 
from yielding to the tremendous resulting centrifugal ten- 
dency. A simple calculation shows that if the earth rotated 
once in about one hour and a third, retaining its shape un- 
changed (which last it could not do unless very much more 
rigid than it is), a body at the equator would be absolutely 
weightless. But a much slower rate of rotation than this 
would suffice to break off the equatorial regions. If the 
earth rotated once in about three hours, the equator would 
increase its distance from the polar axis, the centrifugal 
tendency (the rate of rotation continuing) would be greater 
and the surface gravity less, and the material of the equa- 
torial surface parts would be separated from the rest of the 
earth's substance. 

Dr. Ball follows Mr. Darwin in taking about this rotation 
rate— one spin in three hours — as that existing when the 
moon's mass separated from the earth. If we assume the 
earth at that stage of her existence to have been, apart from 
centrifugal effect, of the same volume and mass as at pre- 
sent, her substance possibly liquid, but not in great part 
vaporous, this estimate would be justified. But it appears 
to me we must not overlook the probability that the separa- 
tion of the moon from the earth took place when a large 

c 



1 8 MYSTERIES OF TIME AND SPACE. 

part of the earth's mass continued vaporous through inten- 
sity of heat. If that were so, the earth's volume would then 
have been much greater than at present, even though her 
mass may have been, as it probably was, much smaller. 
What we see now in the giant planets, long after the moon- 
generating part of their career, seems to confirm this view, 
which a priori reasoning renders probable. We have also 
to take into account the smaller mass of the earth at that 
remote period, before those many millions of years through- 
out which the earth has been gathering year by year hundreds 
of millions of meteoric masses. 

Now, with a larger and less dense orb, a slower rotation 
rate — probably a rotation rate very much slower — would have 
sufficed to cause the earth to part with matter from its equa- 
torial regions, where, of course, the centrifugal tendencies 
resulting from over-rapid rotation would be most pro- 
nounced. 

I have been in the habit, during the last ten years, of 
pointing out when lecturing on the moon that she probably 
had her origin as part of the vaporous or partly vaporous 
mass whence the earth also was formed, and that to this 
origin she owed the peculiar rotational motion which keeps 
the same face ever directed towards the earth. I can see 
nothing in Mr. Darwin's researches which should lead us to 
forsake this, the most natural interpretation of the moon's 
origin ; on the contrary, the vast duration of the past periods 
necessary for the increase of the moon's distance from actual 
contact with the earth to her present orbit, and for the in- 
crease of the terrestrial day from three hours to twenty-four, 
suffices of itself to assure us that the earth at that remote 
time must have been in great part vaporous. The giant 
planets also, as I have already hinted, tell the same story ; 
for though they have thrown off their moons — Saturn, per- 
haps, has not quite finished the work — they are still, as wfe. 
can see from their small density and their aspect, in great 
part vaporous. When they were beginning the work &>f 
moon-formation, many tens of millions of years ago, the^ 



THE VISTAS OF THE PAST. 



19 



were, we may be sure, still hotter, and therefore a much 
larger portion of their mass was vaporous. 

But it is the manner of the moon's birth, as suggested by 
Mr. Darwin (Dr. Ball accepting the suggestion as probably 
sound), which seems to me least likely to accord with the 
probable manner of the moon's generation, and also to 
correspond least with d posteriori evidence. 

Mr. Darwin pictures the earth rotating once in three 
hours, with a double tidal wave (a wave affecting the fluid 
substance of her entire mass), raised by solar action. Such 
a wave, synchronising with what may be called the pulsation 
period of the earth (with the dimensions she then had), 
would get higher and higher, just as a pendulum receiving 
a succession of minute but well-timed impulses swings farther 
and farther, until at length cohesion would no longer be 
possible, and the mass out of which the moon was one day 
to be formed was thrown off. The considerations I have 
indicated above would not affect this reasoning ; they would 
only modify our views as to the size and condition of the 
earth when the moon's mass was thus liberated, and there- 
fore as to the rate of the earth's rotation spin at the time, 
and the period of the moon's first free revolution. But there 
is a more important consideration, now to be taken into 
account, which forbids us, I think, to believe that the 
moon's mass was thus thrown off, as it were, by a single 
effort. The monstrous tidal pulsation which would un- 
doubtedly take place under the conditions described, would 
inevitably lead to the throwing off of a small mass long 
before it had attained swing enough, so to speak, to throw 
off such a mass as the moon's — one eighty -first part of the 
entire mass of the earth. Most probably, too, the crests of 
each tidal wave would throw off a mass of matter at about 
the same time, forming, for the time, two small moons in- 
stead of one large one. Still more probably, in my opinion, 
the crest of each wave would scatter cosmic spray rather 
than a single great globular mass. After each wave had 
thus swollen* and eventually burst into spray, it would 



20 MYSTERIES OF TIME AND SPACE. 

gradually subside for a while, the solar tidal impulses no 
longer quite synchronising with the earth's tidal pulsation ; 
but presently the waves would begin to grow again, would 
flow larger and larger, until again a flight of small masses 
would be flung from the summit of each, Again and again 
the process would be repeated, until at length the earth's 
constantly changing rotation rate would cause the sun's 
tidal action no longer to synchronise with the earth's pulsa- 
tion period. Then, and only then, the earth would cease to 
throw off cosmical spray. 

Now what would be the condition of the matter thus 
thrown off, and what its subsequent behaviour ? Each 
particle, each globule of molten matter, would behave just 
as the moon, according to the theory we are considering, 
has actually behaved. It would begin from the first moment 
of its separate existence to retreat slowly from the earth. 
Long before the tidal wave had again grown sufficiently 
high to throw off spray, the spray last thrown off would 
have passed beyond its reach. Again, each of the tiny 
globules thus thrown off from the earth would at first travel 
nearly in the plane of the earth's equator (later influences 
would modify this relation considerably). Thrown off with 
slightly varying directions and degrees of velocity, the bodies 
expelled on opposite sides at one of these earth-spasms, 
would before long have spread themselves all around the 
earth, some gaining on the main body, -others losing. Pron 
bably before the next flights of cosmical spray left the earthy 
the bodies last thrown off would form a tolerably uniform 
very narrow ring around the earth. j 

This process would have continued between certah 
definite epochs — the first being the time when the earth' 
rotation began to approach to synchronism with her pulsationj 
period, 1 the last being the time when there began to be n( 

1 That is the period of vibration of her mass after any impulsei 
(affecting the whole earth) had been received from without. The earth! 
would as certainly have had such a pulsation period as the vibrating 
substance of a bell has. 



THE VISTAS OF THE PAST, 21 

sufficient approach to synchronism (in the mid-interval only 
would there have been perfect synchronism). This period 
must have lasted for a very long time, probably for millions 
of years. When it was over, what was the condition of the 
matter which had been thrown off from the earth's mass ? 
Manifestly it must have formed at that time a series of close 
concentric rings of tiny satellites. Probably the rings were 
so close that, though each was very narrow, they formed a 
continuous flat and rather broad ring. But, whether this 
were so or not, it is certain that the outermost and inner- 
most ring of the series would form the boundary circles of 
a flat and rather broad ring- system of small bodies, closely 
resembling in appearance (as seen from a great distance) the 
Saturnian ring-system, and having a real structure precisely 
like that which the researches of Benjamin Peirce and the 
Bonds in America, of Clerk Maxwell and others in this 
country, have proved that the Saturnian ring-system actually 
has. 

It seems to me, on the one hand, so clear that the pro- 
cess suggested (with great plausibility) by Mr. Darwin and 
Dr. Ball must really have taken place in such a manner 
as to produce a ring such as I have described ; and, on 
the other hand, it is so certain that the Saturnian ring- 
system is of this nature, that I feel persuaded that we have 
here been led — by paths along two lines of research, each of 
great difficulty, apparently tending in very different directions 
— to the explanation of the mystery of Saturn's rings, and of 
the much deeper mystery of the origin of worlds and moons. 
Sixteen years ago, in the preface to my treatise on 'Saturn 
and its System ' (my first work), I pointed out that probably 
In the study of the Saturnian rings we might find an inter- 
pretation of the manner in which the solar system itself had 
been developed. My prediction, if such it can be called, 
lias not been exactly fulfilled, though the relationship I in- 
dicated between the two problems has been confirmed. For, 
instead of the study of the Saturnian ring-system having 
[ thrown light (except reflected light) on the origin of worlds 



22 MYSTERIES OF TIME AND SPACE. 

and moons, it would seem as though the study of the origin 
of the moon had thrown light on the Saturnian rings. 

Be this as it may, there can be very little question, I 
believe, that the moon was not formed at a single effort, as 
Dr. Ball has suggested, but that a series of rings was first 
formed, constituting a single flat ring-system. The forma- 
tion of the moon from such a system of rings would result 
from the gradual process by which the number of the minute 
bodies forming the ring-system would be reduced by colli- 
sions. If the ring-system was (as seems probable) immersed 
at the beginning, and for a long time, in the vaporous out- 
skirts of the earth, this process would be less slow than it 
otherwise would have been. Satellite after satellite would 
coalesce with neighbouring satellites ; probably, centres of 
aggregation would be formed, which would absorb wander- 
ing satellites in the ring-system still more effectively. Every 
combination of the kind, by changing the period of revolu- 
tion of the mass thus formed (for at every collision there 
would be a loss of vis viva), would tend to hasten the change 
of the ring-system into a single orb. It is no new idea that 
such a process as this took place, no mere attempt to recon- 
cile new results with views previously entertained. The 
occurrence of such changes as I have here described was 
indicated by me sixteen years ago, in my treatise on Saturn 
(p. 126), and it was there shown that changes in the ap- 
pearance of the rings, and probably the recent development 
of the inner dark ring, may be due to processes of this kinjd 
— collisions among the satellites, and consequent loss of vh 
viva by the entire system. 

The formation of the .moon, whether in this manneir, 
which appears to me much the more probable, or as a 
single catastrophic event, must have occurred at so remotje 
a period that the earth's rotation (carrying back over thjf.s 
enormous interval of time the process of retardation, whicjh 
has certainly been in progress) must, when the moon was 
first formed, have been much more rapid than at present. 
The moon's period of revolution, also, must have been verj / 



THE VISTAS OF THE PAST. 23 

much shorter than it now is. From and after that era, the 
processes of change must have been those which Mr. Darwin 
has described, and which Dr. Ball has pictured (with colour- 
ing in some parts perhaps tant soit peu exaggerated). We 
have no occasion to explain, as the latter savant does, how 
the earth's frame recovered from the shock of the moon's 
genesis, or how the scar left on her then plastic surface, 
where the moon's mass had left her, was presently healed 
by the ' gentle ministrations ' of the mutual attraction of the 
particles forming her substance ; 1 for no such scar would 
ever, according to our view, have marred the fair surface of 
the earth. But subsequent changes would have been the 
same in whichever of these two ways — the sudden or the 
gradual — we suppose the moon to have been formed. 

According to either view, it is by no means clear that 
the moon's rotation period would have been the same as her 
period of revolution around the earth, as is now the case. 
But it is certain, that from the beginning of her existence as 
an independent orb, the moon must have been at work in 
raising a tidal wave, and at first far more actively even than 
now. Not only would she have raised a higher wave, be- 
cause nearer to the earth, even had the earth been then what 
it is now ; but since the earth must then have been in great 
part fluid, the moon would from the beginning do what the 
sun had for countless ages been doing — she would raise, like 
him, a tidal wave affecting the whole fluid substance of the 
earth ; and, owing to her much greater proximity, the tidal 
wave she thus raised must of necessity have been very 
,much greater than that raised by the sun. This tidal wave, 
Jike that now raised by the moon, would retard the earth's 
rotational spin, and much more effectively. The retardation 
[of the earth's spin would then, as now, be accompanied by 
la gradual retardation of the moon's motion, and recession 

1 ' By these gentle ministrations, ' says Dr. Ball, ' the wound on 
the earth would soon be healed. In the lapse of time, the earth would 
'become as whole as ever, and at last it would not retain even a scar to 
'testify to the mighty catastrophe.' 

- 



24 MYSTERIES OF TIME AND SPACE. 

of the moon from the earth. And while these changes were 
taking place, the earth, by her attraction on the then fluid 
mass of the moon, would be producing similar effects. The 
moon (supposing her then to have rotated in less time than 
she occupied in revolving round the earth) would be acted 
upon tidally by the earth. A mighty wave of fluid or at 
least plastic matter would circle round the moon in a direc- 
tion contrary to that in which she was rotating ; she would, 
therefore, gradually lose her rotational spin, just as the earth 
was losing hers, only at a more rapid rate. The reaction 
corresponding to this action would be, in the earth's case, 
as in the moon's, shown by increased distance. In other 
words, the earth's rotation and the moon's rotation would 
both be reduced in rate, the moon's the more rapidly, and 
both changes would combine reactionally in increasing the 
distance separating the two bodies. 

Only one of these processes is now going on — the moon's 
action is diminishing the earth's rotational spin (and the 
moon's distance is therefore increasing by reaction), the 
earth's action is not diminishing the rotational spin of the 
moon. The reason why the latter action no longer pro- 
duces any effect is that it has done its work, it has no longer 
anything to work upon. The moon's rotation now synchro- 
nises with her revolution around the earth, there is no tidal 
wave (there could be none if the moon's entire surface were 
covered by ocean, or even if the moon's entire mass wer<p 
fluid), and therefore there is no loss of rotational spin. I 
have said the earth no longer has any work to do so far a;s 
modifying the moon's rotation is concerned. This is nearly 
true, but not quite. The earth has still some work to doj 
-in preventing the rotation rate of the moon from diminish, 
ing, as it would otherwise tend to do, under the sun's action 
If the earth were suddenly destroyed, or rather removed i 
entirely away from the solar system, the moon would con-j- 
tinue to travel around the sun, in a path very little changecji 
from that which she at present follows, and, by such wave 
motion as the sun can produce in the moon's mass, h 



THE VISTAS OF THE PAST. 25 

would tend slowly to diminish her rate of rotation. The 
neighbourhood of the earth prevents any such change from 
occurring, and would do so, even if the sun could raise a 
large tidal wave in deep lunar seas or in the moon's entire 
mass. It will be seen presently that this is a consideration 
of some importance. There is also some work for the earth 
to do — though it is but slight — in diminishing the moon's 
rate of rotation so as to correspond exactly with the slow 
gradual increase in her period of revolution. Students of 
the moon could well wish this were otherwise, so that the 
farther side of the moon, which we never see, might come, 
however slowly, into our ken. 

The earth, then, acting on the moon caused the moon 
to adopt that mode of motion which we recognise in her, 
turning once on her axis while she revolves once around the 
earth. In this peculiarity of the moon's motion we recog- 
nise one piece of evidence, which of itself is absolutely con- 
vincing, as to the vastness of the time-intervals which have 
elapsed since the moon first began her independent exist- 
ence. Whatever the moon's original rotation period may 
have been it was certainly very much shorter than her pre- 
sent rotation period. If we suppose it identical originally 
with her period of revolution there would have been an 
enormous amount of work for the earth to do in gradually 
reducing the period to its present value — both periods, in 
point of fact, simultaneously. We have, then, to carry back 
the earth's history so far that, independently of all other evi- 
dence to that effect, we find ourselves forced to accept the 
conclusion that, at the beginning of the separate existence 
of earth and moon, our earth was a globe rotating much 
more rapidly than at present and much nearer to the moon. 

And here the question arises whether we can find in this 
'.inference any explanation of the undoubted discrepancy 
.between the teachings of geology and those of astronomy 
'ps to the earth's age. On the one hand the study of the 
.earth's crust tells us of one hundred millions of years at the 
(very least during which the earth has been the scene of 



26 MYSTERIES OF TIME AND SPACE. 

changes such as are now in progress, chiefly — one may say, 
altogether — under solar influence. On the other hand, 
regarding the sun's emission of heat as resulting, in the 
main, from the contraction of his mass, we find that, 
assuming his density uniform, or nearly so, the contraction 
of his mass to its present dimensions, even from a former 
infinite extension, would have resulted only in generating as 
much heat as would last, at the present rate of emission, 
about twenty millions of years. We do not gain by sup- 
posing the rate of emission less in former ages of the earth, 
for then, the rate of solar work on the earth being less, the 
length of time necessary to complete the work which 
has actually been done would have been proportionately 
greater. 

The difficulty is very serious. Dr. Croll, who was one 
of the first to call attention to it, suggested the explanation, 
which I take to be inconceivable, that our sun was generated 
by the collision of several orbs which had been rushing 
through space with enormous velocity, and that his supply 
of heat represents the energy of those rushing suns, as well 
as that resulting from compression. My own solution of the 
difficulty is one which is confirmed by other researches, 
including an important investigation by Mr. G. Darwin, that 
the sun is not of nearly uniform density throughout his 
apparent globe, but that he is enormously compressed 
towards the centre, and that, in point of fact, the surface we 
see lies very far above the real surface of the sun. 

Dr. Ball believes that in the former proximity of the 
moon we may find a complete answer to the enigma. In 
the primitive oceans, he says, the moon raised tides as she 
does now, but when she was nearer the tides were much, 
higher than at present. For instance, when the moon'i; 
distance was but forty thousand miles, or roughly, a sixth oT 
her present distance, her tide-raising power was not six: 
times, but two hundred and sixteen (six times six times six') 
times greater than at present. So far Dr. Ball's reasoning; 
is sound ; but I cannot follow him in saying that, therefore^ 



THE VISTAS OF THE PAST 27 

the tides would have been two hundred and sixteen times 
as high as at present. (There is no such simple relation as 
this between tide-producing energy and the height of the 
tidal wave.) Still, we may admit that the tides were very 
much higher then than now. 

'These mighty tides,' says Dr. Ball, ' are the gift which astronomers 
have now made to the working machinery of the geologist. They 
constitute an engine of terrific power to aid in the great work of 
geology. What would the puny efforts of water in other ways accom- 
plish when compared with the majestic tides and the great currents 
they produce ? In the great primaeval tides will probably be found 
the explanation of what has long been a reproach to geology. The 
early palaeozoic rocks form a stupendous mass of ocean-made beds, 
which, according to Professor Williamson, are twenty miles thick up 
to the top of the Silurian beds. It has long been a difficulty to 
conceive how such a gigantic quantity of material could have been 
ground up and deposited at the bottom of the sea. The geologists said, 
" The rivers and other agents of the present day will do it if you give them 
time enough." But unfortunately, the mathematicians and the natural 
philosophers would not give them time enough. The mathematicians 
had other reasons for believing that the earth could not have been so 
old as the geologists demanded. Now, however, the mathematicians 
have discovered the new and stupendous tidal grinding engine. With this 
powerful aid the geologists can get through their work in a reasonable 
period of time, and the geologists and the mathematicians may be 
reconciled.' 

I am disposed to doubt seriously whether mathe- 
maticians and astronomers have done more than to some- 
what alleviate the pressure of the difficulty we are con- 
sidering. That they have subtracted somewhat from the 
work which had formerly been assigned to the sun must be 
admitted. We need not inquire what was the former height 
of the tides, or discuss the action of the tidal wave in detail. 
J If we consider only that the tidal wave, according to the 
,very theory we are considering, has, by its reaction against 
the earth, reduced the earth's rotation-spin from a rate of 
<bne rotation in perhaps not more than three hours, cer- 
tainly not more than six, to one rotation only in twenty- 
four hours, we see that the work done on the earth's crust 



28 MYSTERIES OF TIME AND SPACE. 

must have been enormous. It represents the friction- 
products, so to speak, of all that work. The wonder 
might rather be that the ocean-made beds are not much 
thicker than they are, than that they are so thick. But here 
is our difficulty returning to us in another form. Is it clear 
that the beds considered by Dr. Ball were not made sub- 
sequently to the time when the moon was at the com- 
paratively small distance he mentions? Can we for a 
moment imagine that the tremendous work of checking the 
earth's rotation-spin to less than a quarter of what it was, 
has only left such traces as these? Must not that work 
have been done while still the greater part of the earth's 
mass was fluid, and the water tidal wave have begun its 
work long after ? Geologists have other reasons than the 
thick ocean-made strata for their belief in the vast periods 
of time which form the great difficulty of the problem. 
There is the evidence derived from the study of organic 
matter, the evidence derived from the remains of once-living 
creatures — animal and vegetable. The moon might have 
raised a tidal wave as high as Chimborazo without hastening 
the progress of what may be called the development of the 
earth — nay, she would very seriously have checked this 
progress. It may be doubted, even, whether life, belonging 
to any save the lower forms, could have existed during the 
time when such tidal waves as Dr. Ball pictures careered 
round the swiftly rotating globe. 

It remains to be noticed that, though the day will con- 
tinually increase as the moon recedes, and, vice versa, thte 
length of the month, measured in days, attained long sincje 
its maximum. It was then — some millions of years ago-1- 
about twenty-nine days long, and is now but twenty-seveii 
and one-third days, as days are now. As the moon recedes, 
the lunar month — which is also the moon's day — will contaiik 
fewer and fewer of our terrestrial days. For our days gro\v 
longer, now, at a greater rate than the lunar month iia- 
creases. Our day will continue to grow longer and long&r 
as the moon recedes. In one hundred and fifty millions pf 



THE VISTAS OF THE PAST. 



29 



years, or thereabouts, our day will be about one thousand 
four hundred of our present hours long • this period, also, 
will then be that in which the moon circles around the 
earth — about fifty- eight and one-third of our present days. 
Dr. Ball goes on to consider how the sun would affect this 
state of things. There would be a tide raised by the sun 
on the earth after the moon had ceased to raise any tide 
(the earth's rotation exactly synchronising with the moon's 
revolution) ; and, as a result of this, Dr. Ball says, that the 
earth would begin to rotate in a longer time than the moon 
circles round her. It appears to me that the moon's action 
would check any tendency of this sort, just as the earth's 
action on the moon has, as we know, prevented the moon 
from rotating in a longer period than that of her revolution 
round the earth. The state of compromise with a moon 
circling once in one thousand four hundred hours round 
the earth rotating in the same time, the moon also so ro- 
tating, would be, I believe, a state of stable equilibrium. It 
is not a very pleasant future to look forward to. Fortunately 
it is very remote. 



30 MYSTERIES OF TIME AND SPACE. 



THE BIRTH OF THE MOON. 

Fourteen years have passed since, in the first series of 
my 'Light Science for Leisure Hours/ I discussed the 
change which the tidal wave is slowly but surely producing 
in the length of the day. Certain researches, which had 
then recently been made into the moon's motions, had 
shown astronomers that there must be some force at work 
retarding the earth in her rotational spin. 'In this diffi- 
culty,' I wrote at that time, 'we are not left wholly without 
resource/ We are not only able, I showed, to say that the 
discrepancy between the moon's motions and theory is due 
to a gradual retardation of the earth's rotation-movement, 
but we are able to place our finger on a very sufficient cause 
for such a retardation. One of the most firmly established 
principles of modern science is this, that where work is 
done, force is in some way or other expended. The doing 
of work may show itself in a variety of ways — in the genera- 
tion of heat, in the production of light, in the raising of 
weights, and so on ; but in every case an equivalent force 
must be expended. If the brakes are applied to a train in 
motion, intense heat is generated in the substance of the 
brake. Now, the force employed by the brakesman is not 
equivalent to the heat generated. Where then is the balance 
of force expended ? We all know that the train's motion |is 
retarded, and this loss of motion represents the requisite 
expenditure of force. ' Now,' I asked, ' is there any process 
in nature resembling, in however remote a degree, the appli- 
cation of a brake to check the earth's rotation ? ' ' There is,' 



THE BIRTH OF THE MOON. 



3i 



was the answer ; ' the tidal wave, which sweeps twice a day 
round the earth, travels in a direction contrary to the earth's 
motion of rotation. That this wave " does work " no one 
can doubt who has watched its effects. The mere rise and 
fall in open ocean may not be strikingly indicative of "work 
done ; " but when we see the behaviour of the tidal wave in 
narrow channels, when we see heavily-laden ships swept 
steadily up our tidal rivers, we cannot but recognise the 
expenditure of force. Now, where does this force come 
from? Motion being the great " force-measurer," what 
motion suffers that the tides may work ? We may securely 
reply, that the only motion which can supply the requisite 
force is the earth's motion of rotation. Therefore, it is no / 

mere fancy, but a matter of absolute certainty, that, though ■ / 
slowly, still very surely, our terrestrial globe is losing its 
rotation-movement.' \/ 

The discovery on which this conclusion was based has 
borne notable fruit in recent times. The change which the 
moon's motion was shown to undergo and the change which 
affects the earth's rotation were proved to be alike important. 
They are processes actually taking place, and scarcely any 
process which takes place now fails, when rightly understood, 
to throw light on changes which have taken place in the 
past. Very notably has this proved to be the case in the 
present instance. 

' Let us first briefly sketch the original discovery. Its 
history, carefully studied, affords an excellent lesson in 
showing how important it is for science that even the 
slightest apparent departure from theory should be noted, 
and that, when noted, it should be thoroughly investigated. 

,' When the theory of gravitation was as yet in its infancy, 
iifr the lifetime, indeed, of its great author, a discovery was 
iriade which threatened to invalidate it. Halley, the first of 
NJewton's followers, found that, when the eclipses of the sun, 
w hich are recorded in ancient annals, are examined in detail, 
, le lunar motions necessary to explain them are different 
frfom those of the moon in our own time ; that, in fact, she 



/ 



32 MYSTERIES OF TIME AND SPACE. 

must have moved more slowly in past ages than she does at 
present. 

Ninety years passed before any satisfactory solution was 
offered of the remarkable circumstance thus detected. Then 
the great mathematician Laplace showed how the moon's 
movements are in reality being hastened on account of a 
change which is taking place in the form of the earth's orbit. 
The moon travels round the earth * under the action of ter- 
restrial attraction ; but the sun, though much more remote 
than the earth, largely influences the moon's motion. In- 
deed, the sun is, in reality, the moon's chief guiding power. 
In regard, however, to her motion considered in reference 
to the earth, the sun has only a subordinate influence. This 
influence tends on the whole to diminish the earth's power 
on the moon, so that the latter travels in a wider orbit and 
more slowly than she would but for the sun. The nearer 
the earth to the sun, the greater is the sun's power to 
diminish the earth's sway, and the more slowly does the 
moon move. In December and January, for example, the 
lunar month is longer than in June and July, the earth being 
nearest to the sun on about January i and farthest from him 
on about July i. Now, the eccentricity of the earth's orbit 
is at present, and has been for many years, diminishing, her 
path becoming more and more nearly circular (though i'; 
will never become actually circular, the eccentricity attaining 
a minimum after a certain long period of time, and theili 
gradually increasing). As the longer axis of the path re- 
mains unchanged, it follows that the area enclosed by the 
earth's path is gradually increasing, so that on the whole th/e 
sun's perturbing influence is diminishing. Since this in- 
fluence acts to increase the moon's distance and diminish 
her rate of motion, it follows that, as the influence diminishes, 
the moon's rate of motion increases. As the eccentricity of 

1 In reality the moon travels round the sun, and is in that motion 
largely perturbed by the earth. But considered with reference to trite 
earth as centre, the moon travels round the earth, and is in such modem 
perturbed by the sun. \ 



THE BIRTH OF THE MOON. 33 

the earth's orbit has been diminishing throughout all the 
time over which astronomical records extend, it is clear that 
the hastening of the moon's motion discovered by Halley 
may find its explanation in this change. 

Laplace supposed that this was actually the case. His 
investigation of the so-called acceleration seemed to be no 
less exact than profound. The calculated acceleration agreed 
so closely with what observation appeared to indicate, that 
science was supposed to have achieved a great triumph, and 
the law of gravitation, which for a time had seemed shaken 
(at least, it seemed as though some hitherto unknown forces 
must have been at work), was placed on a sounder basis 
than ever. 

But our great astronomer and mathematician Adams, 
having re-examined this question twenty years or so ago, 
discovered a notable flaw in Laplace's reasoning. He found 
that disturbances which Laplace had supposed he might 
neglect, were in reality important. Laplace had considered 
slow variation in the sun's action in diminishing the earth's 
pull on the moon, but he had regarded as probably insensible 
the slow variation taking place all the time in the sun's direct 
action on the moon. When all the disturbing forces were 
duly taken into account, it was found that the calculated 
acceleration of the moon's motion amounts to only about 
half what Laplace had made it. The observed acceleration 
then, which was very satisfactorily explained so long as 
Laplace's results were accepted, was found to be but half 
accounted for. The other half had still to be explained. 

It was then that Delaunay and others advanced the 
theory that perhaps the so-called acceleration of the moon 
may in part be apparent only. We measure the movements 
of the celestial bodies by our earth's, taking as our unit of 
time-measurement the period in which our earth rotates once 
on her axis, or what is called a sidereal day (the solar day is 
foiur minutes longer, and changing all the time). This had 
b<!en regarded as absolutely constant. It is the basis not 
oAly of celestial motion-measurements, but of our entire 



"34 MYSTERIES OF TIME AND SPACE. 

system of weights and measures. But Delaunay suggested 
that our great terrestrial timepiece may be losing and may 
be running slow — not only losing time day by day as 
compared with the time she gave thousands of years ago, 
but losing rate, so as to go more and more slowly as time 
goes on. 

But let us see on what grounds all these investigations 
had proceeded, and what was the real observed peculiarity 
of the moon's motion about which so much inquiry had been 
made. The moon is apparently moving more quickly now 
than she was 2,000 years ago ; but suppose astronomers 
timed her now, and that, with all their acquired knowledge 
respecting the acceleration due to the cause indicated by 
Laplace, they calculated her position day by day among the 
stars for the next thousand years. As time went on, how 
much would the moon seem to gain by that remaining part 
of the acceleration which has not been accounted for ? Well, 
at the end of the thousand years, she would not quite have 
gained half her own apparent diameter. Twelve hundred 
years would have to pass before her centre would be where, 
according to calculation, her forward edge should have been. 
In reality, it would be not the moon which would have gained 
so much, but the earth which would have lost so much. In 
twelve hundred years the earth's spin would be less than It 
should be if there were no change by the amount of rotation 
corresponding to what would carry a line from the earthrs 
centre over half the breadth of the moon's face. That would 
be about a quarter of a degree, or, roughly, about a 1,400th 
part of the entire circuit of the heavens. As the moon com- 
pletes the circuit of the heavens in twenty-seven days aid 
a third, the difference in time would therefore be, roughly, 
about a 1,400th part of this, or some twenty-eight minutas. 
Looking back instead of looking forward, let us consider 
what the earth, regarded as a timepiece, has lost during tl'ie 
two thousand years which have elapsed since the earliest 
eclipses of which we have exact records. ' Suppose/ I wroke 
fourteen years ago, ' that the earth was then timed and rateld, 



THE BIRTH OF THE MOON. 35 

how much has she lost, and what is her " rate-error " ? She 
has lost in that interval nearly an hour and a quarter, and 
she is losing now at the rate of one second in twelve weeks. 
In other words, the length of a day is now more by about 
one eighty-fourth part of a second than it was two thousand 
years ago.' 

Such was the minute, one might have supposed the in- 
appreciable, change in which was to be found the secret of 
some of the most important cosmical phenomena. In this 
slow change, minute in itself, and detected by its action in a 
period of time which, though long compared with man's life, 
is ,the merest nothing compared with the eras of a world's 
lifejtime, science was to find an explanation of changes in the 
past so stupendous that the mind can hardly realise them ; 
the. indication of changes so great in the future, that when 
the ( y have occurred life on this earth will no longer be pos- 
sible (at any rate, to creatures resembling any now existing 
on ifhe earth's surface). 

-The result thus far noted is that owing to the tidal action 
OjT the moon the length of the day is increasing. We may 
lctak forward into the remote future for a time when the day 
w ill be twice, or thrice, or four times as long as at present. 
Cj>r we may carry back into remote depths of past time the 
cjhange thus taking place, and recognise an epoch when the 
ejjarth rotated in half the time, another when she rotated in 
a) quarter of the time, in which she now rotates. But before 
\v,le do this we have to ask what becomes of the rotational 
eihergy which is thus being lost ; for whatever effects we find 
accompanying this change must also be traced forwards and 
plek. 

j- Here is our spinning earth losing her spin in consequence 
ofj f the moon's action. It may be that the loss of energy 
thj-ius indicated is entirely compensated by the heat resulting 
frcnm the frictional action on which the diminution of the 
re] cation rate depends. In that case we need not look else- 
w.'Jiere for any counter effects. But we may at the outset 
se/, e that some of the results of tidal action on the earth must 

d 2 



36 MYSTERIES OF TIME AND SPACE. 

produce counter-effects outside the earth. If we imagine 
the great tidal waves set in motion once for all around the 
earth, and gradually retarding by their frictional action the 
earth's rotation, we know that besides the loss of rotation, 
and besides the generation of heat, there would be another 
observable effect — viz., the gradual dying out of the tidal 
wave. Now, the tidal wave being maintained from without 
by the lunar action (we may leave out of consideration for 
the moment the sun-raised solar part of the tidal wave), we 
see that one of the counter- effects which, but for the external 
action, would accompany the frictional retardation of the 
earth's rotation is not taking place. We have to seek for 
some other counter effect. As there is none on the earth, 
we may take it for granted that some such effect exists in 
the motions of the moon, the orb which is concerned with 
the earth and the sea in the tide-raising action. 

Now, analysis of the matter by strictest mathematical 
investigation, in the able hands of Mr. George Darwin, 
Fellow of Trinity College, Cambridge (son of the great 
Charles Darwin), has shown that this effect must be a gradual 
diminution in the rate of the moon's motion, accompanied 
by an equally gradual increase in the moon's distance. The 
reader must not expect that the reasoning can be made clear 
to him in such a paper as the present. All these questions 
of the action and interaction of rotating and revolving bodies 
require for their discussion the profoundest mathematical 
knowledge, as well as that which such knowledge of itsejlf 
indeed implies, the keenest mathematical insight. When' I 
notice that even so skilful a mathematician as Sir George 
Airy, in a mathematical investigation especially relating ,to 
the moon's tide-raising action in diminishing the earth's 
rotation-spin, had actually announced that there is no suih 
influence, only at the last moment detecting, amongst trjie 
complex formulae involved, the presence of terms which, 
duly developed, indicate such action, 1 it will be seen h(bw 

1 Another remarkable illustration of the difficulty of all su":h 
investigations is to be found in the rejection by Leverrier and Ponte- 



THE BIRTH OF THE MOON. 37 

utterly inadequate must be the discussion of the matter on 
ordinary mechanical principles to indicate to the general 
reader the necessity of the change in the moon's motion and 
distance demonstrated by Mr. Darwin. 

Mathematical analysis shows unmistakably, however, 
that the moon's distance must continue to increase while 
the earth's rotational motion continues to diminish. These 
processes take place continuously, though not at an un- 
varying rate. We can carry them back to a beginning, and 
forwards to an end. Nor does science know of any cir- 
cumstance in the past or in the future of the earth which 
should prevent us from carrying each process to its extreme 
limit either way ; in other words, there is no scientific 
reason for believing that the earth and moon began their 
existence at some one of the stages to and through which 
we can trace the processes of change in the past, or that the 
continuance of these processes in the future will be suddenly 
brought to an end before they have completed their work. 
As to the past, indeed, science has very strong evidence to 
show that these processes started into action at the very 
beginning to which they can theoretically be traced. Just 
as from the study of a tree, an experienced gardener can 
tell that it grew from the seed, and not from a slip or 
cutting, so the astronomer and geologist can infer, from the 
evidence presented by the earth and moon, that they started 
into separate existence from the primordial planetary state, 1 
and not more fully fashioned as companion planets. 

Our retrospective view into the vistas of long past time 
encounters no obscuring mist till we reach a time when 
earth and moon formed one mass. How long ago this may 

conlant of the results obtained by Adams, as mentioned above, Ponte- 
coulant even denouncing Adams's method of treating the subject as 
analytical legerdemain [supercherie analytiqtie). 

1 We mean by the words 'primordial planetary state' to distinguish 
the earliest condition of planets whereof we have scientific evidence 
from the absolutely primordial condition of matter, of which science 
knows nothing. 



38 MYSTERIES OF TIME AND SPACE. 

have been we do not know. It must have been more than 
fifty million years ago, if we can at all trust the astronomical 
and geological evidence. Probably it was a good deal more 
than ioo million years ago. It may have been farther back 
still. We see it in the dim vista of the past, but it looms in 
such sort that we cannot estimate its real distance. We are 
more apt to think it nearer than farther than it really is. I 
will not follow Dr. Ball in citing apt illustrations of similar 
doubts, even in matters historical. It must be evident that 
when we have to deal with processes operating on so large 
a scale, and requiring such enormous periods of time, there 
can be nothing in our experience enabling us even to ap- 
proximate to the exact time intervals. We must be content 
to say that they are measurable by tens of millions of years, 
but how many such tens of millions of years they include 
we cannot tell. 

The critical epoch to which we look back may be 
regarded as the time of the moon's birth. Diminishing 
in imagination the period of the earth's rotation, we find 
gravity amply competent to keep the earth's mass together 
against the resulting centrifugal tendencies as the day passes 
down to half a day, to a quarter of a day, and even to a 
much less period. But it is evident there must be a limit 
to this. Suppose the earth were rotating at such a rate, for 
instance, that gravity vanished at the surface of the equator. 
Then it might at first seem as though, with such a rate of 
rotation, the equatorial parts would simply remain as they 
are, since gravity being just balanced by, and just balancing, 
centrifugal force, there would be no tendency in the equa- 
torial parts to separate from the rest of the earth. IJ3ut 
a little consideration will show that, on the contrary, w;th 
this rate of rotation, the earth must inevitably fly to pieces 
like a grindstone set in too rapid rotation. For there beijag 
no pressure at the equator, and very little pressure through- 
out the neighbourhood of the entire equatorial plane of r'he 
earth, whereas near the poles and along the neighbourhood 
of the polar axis there would be great pressure, and eRse- 



THE BIRTH OF THE MO OH. 39 

where a pressure increasing as the polar axis was ap- 
proached, it follows inevitably that these pressures not 
being balanced in the neighbourhood of the equatorial 
regions, these must be forced outwards. At an increased 
distance from the polar axis the rotation of these masses in 
the short period mentioned would produce a centrifugal 
tendency exceeding the force of gravity. They must, there- 
fore, either separate from the rest of the earth, and not 
attain the same rapid rotation as the rest, or, attaining it, 
they must be separated by being thrown outwards from the 
centre. In either case there would be a separation of the 
equatorial masses from the rest of the earth, if the rotation 
took place in an hour and twenty-four minutes. This rate 
of rotation would in fact be much too great for cohesion. 
Calculation shows that, assuming the earth of the same mass 
as at present, and taking such a constitution of its interioi 
as seems fairly probable in the time of the earth's fluidity 
through intense heat, a rotation once in about three hours 
would have been the most rapid which could have existed 
without the separation of the equatorial parts of the earth 
from the rest. 

I may note that, although it may be considered by many 
a cautious mode of procedure to take no account here of 
the possibility that, at the remote time when the moon was 
born, the earth may have had a much smaller mass than at 
present, or again, to consider the possibility that at that 
time a great portion of the earth's mass may have been 
vaporous, I do not myself recognise extreme caution in this, 
but excessive daring, or rather wild recklessness. Con- 
sidering the multitudes of meteors which fall even now, 
after many tens of millions of years during which the 
meteoric supply of the solar system has been undergoing a 
process of exhaustion, it appears to me we are as much 
bound to trace back the process of meteoric downfall which 
wie know to be constantly taking place, as to trace back 
tfyat slow change in the length of the day, and in the dis- 
tance of the moon, which mathematical analysis shows to 



40 MYSTERIES OF TIME AND SPACE. 

be in progress. We cannot escape the conclusion that in 
past ages the process took place at a much greater rate 
than at present. We may fairly enough believe that when 
the earth was in its vaporous condition it was very much 
larger, though much less massive than now. The conclusion 
is absolutely inevitable that in each circuit around the sun 
the earth captured then a much larger number of meteoric 
masses than at present ; for then they were much more 
richly strewn, and the earth herself was much larger. Even 
now it is calculated that she captures three or four hundred 
millions of meteors of all orders in the course of a year. 
Then, it seems no rash inference, she captured hundreds, 
or perhaps thousands, where she now captures one. Such 
a process taking place during fifty or perhaps a hundred 
millions of years, cannot but have added enormously to the 
mass of the earth. It is certainly most unsafe to neglect a 
process which assuredly took place, and as certainly must 
have had a large share in modifying the conditions under 
which the processes, considered by Mr. Darwin, took place. 
If we take, however, no notice of what nevertheless is 
certainly a most important point in the problem, we go back 
to the time when the earth, having something like its pre- 
sent mass, and being in a partial or wholly fluid state, with 
a volume not differing greatly from its present volume, 
rotated on its axis in about three hours. All the evidence 
we have shows us that the earth, at the remote epoch to 
which we are thus led, must have been very different in all 
respects from what she is now ; the giant planets which are 
nearer to that earlier stage of planetary existence, are very 
unlike the earth ; the sun, which is still younger, so far as 
development is concerned, differs from her still more ; but 
a judicious scientific caution leads us to disregard evidence 
of this sort, and to assume what certainly is not the case 
rather than to make any allowance for changes whose exa<:t 
amount we are unable to estimate : this is so much the 
more judicious that the whole problem is one in which tfte 
elements are doubtful to a greater or less degree. To speak 



THE BIRTH OF THE MO OH. 41 

seriously, no investigation of the problem attacked by Mr. 
Darwin can be regarded as sufficient, which does not take 
into account — (1) the probable gaseity of a large portion of 
the earth's globe at the time when the moon's mass was 
separated from hers; (2) the consequent small mean density 
of the earth, or, which is the same thing, her large volume 
(as compared with her mass) ; (3) the circumstance that no 
small portion of the earth's present mass must have been 
added by meteoric downfall since the time when the moon's 
mass was separated from hers, the moon having also gained 
greatly in mass since that remote epoch. l 

With this proviso, which in no sort affects the general 
inference deducible from Mr. Darwin's reasoning, though it 
very largely affects our estimates of the time-intervals corre- 
sponding to the changes which we have to contemplate, we 
resume the study of his conclusions. 

At some time, then, very far back in the remote past, 
and when the earth was rotating much more rapidly than at 

1 Assigning to the earth, then, a volume — owing to smaller density 
— eight times her present volume, and therefore a surface four times as 
great as at present, regarding those meteoric members of the -solar 
system which were capturable (because of the position of their orbits) 
by the earth as 1,000 times as numerous as at present, and diminishing 
uniformly in richness of distribution to the present time, setting that 
epoch 100,000,000 years before the present time, and regarding the 
average weight of meteors in Prof. Newton's calculations (by which 400 
millions of all sorts reach the earth each year) as only 10 oz. (a fair 
enough allowance when some single ones weigh hundreds of pounds), 
the total number of meteors which have fallen on the earth would be : 

4 x 1J >£- x 100,000,000 x 400,000,000 x 10 oz. 

= 800,000,000,000,000,000,000 oz. 

= 50,000,000,000,000,000,000 lbs. 

= 22,322,000,000,000,000 tons. As the estimated 

mass of the earth at present is about 6,000,000,000,000,000,000,000 
tons, this may seem but a small aliquot part, yet even as thus estimated 
it is a great deal too large to be neglected. Considering that Bischoff 
assigns 350 millions of years to the period during which the earth has 
coded from 2000 C. to 200 C, it will be tolerably obvious that we 
have much underrated the earth's growth. 

I 



s 



42 MYSTERIES OF TIME AND SPACE. 

present, the mass subsequently to form the moon was free to 
separate itself from the earth, or was already separate, though 
close to the earth, or was compelled to separate itself; it is 
not easy to say which view of the three we should adopt. 
The mass may have been a separate ring, or a single body, 
or far more probably a ring of small bodies. 

Dr. Ball is careful to show how the scar left when the 
moon's mass was separated from the earth's gradually closed 
up, and eventually disappeared. ' I can easily imagine,' he 
says, ' an objector to say: " If the moon were merely a frag- 
ment torn off, how can we conceive that it should have that 
beautiful globular form which we see ? Ought not the moon 
to have rugged corners and an irregular shape ? and ought 
not the earth to show a frightful scar at the spot where so 
large a portion of its mass was rent off?" You must re- 
member,' he proceeds, in reply to this imagined objection, 
' that in those times the earth was not the rigid solid mass 
on which we now stand. The earth was then so hot as to 
be partially soft, if not actually molten. If, then, a fragment 
were detached from the earth, that fragment would be a soft 
yielding mass. Not for long would the fragment retain an 
irregular form ; the mutual attraction of the particles would 
draw the mass together. By the same gentle ministrations 
the wound on the earth would soon be healed. In the lapse 
of time the earth would become as whole as ever, and at 
last it would not retain even a scar to testify to the mighty 
catastrophe.' 

I believe that the separation of the moon from the earth 
took place under conditions very different from those here 
considered : that no irregular mass was torn off from the 
earth, no ragged gap remained at the place whence the 
moon's substance had been removed. It seems to me i;m- 
possible to conceive any process of steady change which 
could have culminated in the imagined catastrophe. Mr. 
Darwin considers, indeed — and very likely he is right 
(though the idea is, as he admits, a mere speculation) — triat 
the tide raised by the sun's influence in the mass of tihe 



THE BIRTH OF THE MOON. 43 

rotating earth, may have led to the separation of the moon's 
mass. But this would not have happened, if it happened at 
all, as a single event. Remembering that the time when 
this action would he most effective would be the time when 
no part of the earth's globe was solid — and that stage of 
the earth's history must have lasted for millions of years — it 
is obvious that the tidal swing might very well have increased 
(in the manner suggested by Mr. Darwin) until some portion 
of the top of the tidal wave was left outside the earth. But 
the portion thus left would be but small, and immediately 
after the tidal wave would be reduced in height. (There 
would probably be two masses thus, as it were, thrown off, 
one from each of the two opposite tidal waves.) Gradually 
after this the wave would increase again in height, and again 
the undue elevation would result in the throwing off of 
matter from the top of the great tidal wave. The process 
would be repeated again and again, each mass thus thrown 
off slowly retreating, owing to the action of forces similar to 
those which cause the present slow retreat of the moon. 
The bodies thus thrown off would form a flat ring in the 
plane of the earth's equator — a ring probably similar to 
that which now surrounds the planet Saturn. It is to all 
intents and purposes certain that the mass which was 
eventually to form the moon's mass was thrown off in this 
gradual manner. When we see in the Saturnian system a 
ring precisely like that which must thus have been found 
around the earth, we are justified in finding ' confirmation 
strong ' of the theory as to the moon's formation which Mr. 
Darwin has advanced. 1 

Mr. Darwin himself, indeed, considers that the moon's 

1 I may note here that in the preface to Saturn and its System (the 
first book of my writing) there occurs the following passage, in which I 
made a prediction, veiy strikingly confirmed, I think, by the relations 
above indicated : ' It is not impossible that in the variations perceptibly 
proceeding in the Saturnian ring-system a key may one day be found 
to 1 the law of development under which the solar system has reached its 
present condition.' 



44 MYSTERIES OF TIME AND SPACE. 

mass was thrown off at a single effort as it were. The 
reasoning relating to this part of his views does not belong, 
indeed, like the rest, to the sure domain of mathematics, 
but to speculation. Let us, however, follow it as presented 
by an astronomer who apparently accepts the reasoning as 
sound — Dr. Ball : — ' One hint,' he says, ' dynamics does 
give. It reminds us that a rotation once in three hours is 
very close to the quickest rotation which the earth could 
have without falling to pieces. As the earth was thus pre- 
disposed to rupture, it is of extreme interest to observe that 
a cause tending to precipitate such a rupture was then ready 
to hand. It seems not unlikely that we are indebted to the 
sun as the occasion by which the moon was fractured off 
from the earth and assumed the dignity of an independent 
body. It must be remembered that the sun produces tides 
in the earth as well as the moon J (that is, as the moon does). 
' The solar tides are small compared with the lunar tides 
. . . but before the moon was detached the earth was dis- 
turbed by the solar tides alone. The primaeval earth thus 
rose and fell under the tidal action of the sun. Probably 
there were no oceans then on the earth \ but tides do not 
require oceans or even water for their operation. The 
primitive tides were manifested as throbs in the actual body 
of the earth itself, which was then in a more or less fluid 
condition. Even at this moment bodily tides are disturbing 
the solid earth beneath our feet ; but these tides are nojw 
so small as to be imperceptible when compared with tine 
oceanic tides. . . . Suppose now that the liquid primseyal 
globe were pressed in on two quadrants and drawn oiut 
on the two others, and that the pressures were then ire- 
leased. The globe would attempt to regain its origiihal 
form ; but this it could not do at once, any more thani a 
pendulum can at once regain its vertical position ' (after beiiig 
swung) ; ' the protruded portions would go in, but thtjby 
would overshoot the mark, and the globe would thus oscil- 
late to and fro. Now, it has been shown that the period (of 
such oscillations in our primitive globe is about an hour amd 



THE BIRTH OF THE MOON. 45 

a half, or very close to half the supposed length of the day 
at that time. The solar tides, however, also have a period 
half the length of the day. Here, then, we have a succession 
of small impulses given, which are timed to harmonise with 
the natural vibrations. The solar tides raised threw the 
earth into large vibrations. At first these were small, but at 
each succeeding impulse the amplitude was augmented until 
at length the cohesion of the molten matter could no longer 
resist : a separation took place : one portion consolidated 
to form onr present earth ; the other portion consolidated to 
form the moon.' 

It is not safe to assert what would or would not happen 
under conditions utterly unlike those with which we can deal 
in actual experiment. But so far as I can judge from all the 
known properties of matter, I am led to believe that no such 
wave as is here considered by Dr. Ball, and as Mr. Darwin 
had already indicated as likely to arise, could by any pos- 
sibility come into existence. Long before it had attained 
anything like such dimensions as this theory of moon gene- 
ration requires, cohesion between its parts must have ceased. 
Consider the mere increase in the centrifugal force in the 
crest of such a wave, and it will become apparent that when 
the earth was rotating at the critical rate which barely kept 
its equatorial parts together, a much smaller vibrational rise 
would result in separation. It seems to me as absolutely 
certain as anything not falling within the domain of actual 
experiment and observation can possibly be, that the great 
tidal vibration would break into spray long before it reached 
such a height that the portion free to separate was compar- 
able in mass with our own moon. 

We may note, too, that, while the appearance of the 
Saturnian rings corresponds with the manner of moon gene- 
ration here considered, which in itself is a strong argument 
in favour of the theory, we may view the Saturnian rings in 
another way which even more strongly suggests that this is 
the actual way in which moons are born : — 

We see in the Jovian system four moons, all fully formed, 



46 MYSTERIES OF TIME AND SPACE. 

the innermost still very near to its parent orb ; and we find 
Jupiter himself in a condition, judging by his mean density, 
corresponding to what we may suppose to have been the 
earth's condition when the moon, after being fully fashioned, 
had receded to a corresponding distance (that is, to some- 
thing like the same relative distance from the earth). Mr. 
George Darwin has, indeed, shown in a very interesting dis- 
cussion of Jupiter's compression and the motions of his 
moons, that from something more than the mean density of 
the planet this condition of Jupiter may be inferred. It is 
as nearly demonstrated as such a relation can well be, that 
the central part of Jupiter is greatly compressed, compared 
with the outer parts of the planet as we see him. Now 
Saturn, judged by his mean density, appears to be in an earlier 
stage of his career as a planet than Jupiter. While Jupiter's 
mean density is but a fourth of the earth's, Saturn's is barely 
one-seventh of hers. We may somewhat safely infer, then, 
that Saturn's moon-generating work is not so far advanced 
as Jupiter's ; and, in fact, considering his moons only, we 
see that this really is so, for his nearer moons are much 
closer to him, absolutely as well as relatively, than are those 
of Jupiter. It is true Japetus, the outermost Saturnian 
moon, has receded to a greater absolute distance than the 
outermost moon of Jupiter, and Saturn being a smaller 
planet than Jupiter, this greater absolute distance of Japetus 
implies a relative distance greater in still higher degree. But 
this only serves to show that, from whatever cause, /the 
moon-generating process in the case of the Saturnian system 
has progressed more slowly than in the case of the Jovian 
system. It is at any rate clear that the innermost of Saturn's 
moons is relatively much younger than the innermost of 
Jupiter's. This being so, what opinion are we to forrh of 
the ring-system ? Does it not, on the face of matters, ap pear 
as though this ring-system represented an embryo moon, or 
perhaps the embryos of several moons ? Finding thus aropnd 
the planet of least density (presumably, therefore, the >one 
which has advanced least towards its final condition), ) the 



THE BIRTH OF THE MOON. 47 

planet which has the nearest moons, and, in fine, the planet 
which — if such planet there is — must be regarded as alone 
in the moon-generating stage of planetary existence, this 
singular appendage, absolutely unique in the solar system, are 
we not justified in saying that here we see the last stages of 
the moon-producing stage of a planet's life ? It seems to me 
that this is the most probable interpretation of the rings — 
if it be not the only interpretation available. If we accept it, 
we see what a moon is like when as yet not fully fashioned. 
It consists not of a single globe, not of several large globes 
one day to condense into one, but of rings of multitudinous 
bodies, strewn so closely that, from a distant observing- 
station, they appear to form continuous solid or liquid rings. 
If this were an interpretation of the Satumian rings to which 
the discussion of our own moon had led us, a certain degree of 
hesitation might be suggested by the circumstance that pos- 
sibly our interpretation so deduced might be a little forced. 
But the reverse of this holds — we are encouraged to adopt the 
view instead of being led to doubt it — when we note that more 
than seventeen years since, the Satumian rings were proved 
to be constituted in the manner here described. Nothing 
can be much more complete than the demonstration of this 
which was given by the Bonds and Prof. B. Peirce in 
America, and Prof. Clerk Maxwell in England. There can- 
not now be a shadow of doubt that the entire Satumian 
ring-system consists of discrete satellites, as the sands of the 
sea-shore for multitude, richly aggregated in some parts of 
the system's breadth, sparsely strewn in others. Now, this 
being presumably an embryonic moon-system — perhaps to 
form one moon, perhaps to form several — we have strong 
evidence in favour of the belief that a moon is thrown off 
from the parent planet in this form, and not as a single body; 
for certainly there is no reason for supposing that the pro- 
cess of moon-formation in Saturn's case would be different 
from the corresponding process in the case of any other 
planet. 

We may draw yet another inference from the giant 



48 MYSTERIES OE TIME AND SPACE. 

planets as to the past of the earth and moon. So far as we 
can judge from Jupiter and Saturn, a planet remains in a 
partly vaporous, partly fluid state long after the moons have 
been formed, and have receded to a great distance from the 
parent orb. We may perhaps assume, indeed, that it is 
while a planet is in such a state that the forces thrusting a 
moon away from its parent planet's neighbourhood are most 
active. Unquestionably, when a large part of a planet is 
fluid, so that a great wave of fluid or plastic matter circuits 
around the planet, while what is one day to become the 
planet's ocean exists only in the form of steam or cloud or 
falling rain- showers, the forces at work checking the rotation 
of the planet, and pari passu repelling the infant moon, 
would be far more active than when the chief retarding agent 
was an oceanic tidal wave. Combining with this the con- 
sideration that when the moon was nearer its tide-raising 
power was greater — not as the inverse square, but as the 
inverse cube of the distance — we must attribute to the earlier 
stages of a moon's independent career the greater part of its 
work in checking the rotation of its parent planet, and 
thus (indirectly) causing its own repulsion from that body's 
neighbourhood. j 

Thus I cannot for my own part consider that mud* of 
the work done by the tidal wave in forming the earth's crust 
was effected, as Dr. Ball believes, when the moon was much 
nearer to the earth than now. That within the rangef of 
time over which the geologic record extends the mopn's 
action was much more effective than it is at present, we :may 
well believe ; but that, at any time while the earth's fo^sili- 
ferous strata were being formed, the moon was within 40.1 000 
or even 100,000 miles from the earth I cannot regard as 
likely, or even credible. If the long time-intervals necessary 
to explain the features of the earth's crust could be greatly 
shortened by such considerations as Dr. Ball has eloquently 
urged, the case would, perhaps, be different. There 4s an 
enormous difficulty, unquestionably, in reconciling the vast 
period (ico millions of years at least) during which the earth 



THE BIRTH OF THE MOON. 49 

seems to have been acted upon by the solar rays as at present, 
with the comparatively short period (not more than twenty 
millions of years) during which the sun can have done such 
work as at present, if his emission of heat is regarded as 
solely due to his contraction to his present dimensions. But 
we cannot evade the difficulty by appealing to the moon's 
former tide-raising energies. There are other lines of argu- 
ment besides Dr. Croll's by which the vastness of the period 
during which the sun has worked as he does now in the 
emission of heat and light can be demonstrated. Either our 
interpretation of the source of his heat is incorrect (or at 
least incomplete), or else, as for my own part I believe, the 
process of solar contraction has gone much farther than 
those infer who imagine that the sun's real globe is nearly ' 
of the dimensions of that orb which is bounded by his pho- 
tosphere or light-surface. But be this as it may, there can 
be very little doubt that when the moon was but 40,000 
miles from the earth's surface, there was no life on the earth, 
no surface which could support life. In all probability she 
was, in the same state as Jupiter— her surface so hot that 
the " waters which were one day to form her oceans were 
kep constantly by intensity of heat in the form of vapour, 
savt^ where, at a great height from the fiery surface below, 
they were condensed to the form of visible clouds. 

As regards the future of the moon, in which is involved 
to some degree the future of the earth, we may accept the 
general conclusions of Mr. Darwin and Dr. Ball, though the 
estimate of the time-intervals which must elapse, ere the 
succ essive changes are reached, cannot be regarded as trust- 
worthy. (The problems involved are far too complex to be 
satisfactorily dealt with in the present stage of the discus- 
sion : I doubt even whether science will have ascertained, 
a thousand years hence, the true rate at which the moon's 
recession will take place during the next ten millions of 
years.) 

In the first place it is to be noted that the terrestrial day 
is now shortening more quickly (we ought rather perhaps to 

E 



50 MYSTERIES OF TIME AND SPACE. 

say less slowly) than the lunar month is • lengthening — so 
that, though the month is lengthening, the number of days 
it contains is gradually diminishing. It was otherwise in the 
past. The number of days in the lunar month continually 
increased until the time when the month lasted about 
twenty-nine days, since which time the number of days in 
the month has continually diminished. Dr. Ball describes 
the time when the number of days- in a lunar month was at 
its maximum as the time when the month was in the zenith 
of its glory, — why, this deponent sayeth not, not knowing. 
Measured in any other way than by terrestrial days, the 
month grows constantly longer, and will do so until the 
moon no longer has any work to do in retarding the earth's 
rotation. This is the same as saying that the lunar month 
will continue to lengthen as long as it differs from the 
terrestrial day. Thus, great as the period would be during 
which the day would have to lengthen to equal the present 
lunar month, we have to look forward to a still greater 
distance in the remote future for the time when the length- 
ened day, and the less lengthened lunar month, will be 
equal. At that time the day and the month will each last 
1,400 hours, as hours are now measured, or 58 J of our present 
days. Dr. Ball puts the time when this change will have 
been effected 150,000,000 years from the present time.-' It 
appears to me his estimate falls far short of the truth. The 
actual lengthening of the day, noted since the timje of 
Hipparchus, has accrued at a much slower rate than Dr. 
Ball's estimate would imply. However, even if the epoch 
be no more remote than this, we need not fear tha' the 
progress of the change will seriously affect either ourselves 
or our descendants for many generations to come. Projoably 
long before ten millions of years have elapsed, much 'more 
important changes will have affected the earth — as loss 
of solar heat, the effect of long-continued internal cHanges, 
such as are now in progress, and so forth. We may be as 
easy respecting the lengthening of the terrestrial day, on 
account of the great remoteness of the final condition, as 



THE BIRTH OF THE MOON. 51 

we may be respecting catastrophes threatened as nearer at 
hand — on account of their improbability. x 

Admitting the possibility that, at the remote epoch when 
the change has been effected, there may be reasoning beings 
upon this earth, we may accept the fanciful ideas suggested 
by Dr. Ball. ' Our remote posterity/ he says, ' will have a 
night 700 hours long, and when the sun rises in the morning, 
700 hours more will elapse before he can set. This,' he 
adds (though we should suppose he can hardly be very 
confident on this point), ' they will find a most suitable and 
agreeable arrangement. They will look back on our short 
periods of rest, and short periods of work, with mingled 
curiosity and pity. Perhaps they will even have exhibitions 
of eccentric individuals able to sleep for eight hours, work 
for eight hours, and play for eight hours. They will look 
on such curiosities in the same way as we look on the man 
who undertakes to walk a thousand miles in a thousand 
hours.' ('All which propositions,' as Carlyle words it, 'I, 
for the present, content myself with modestly but perempt- 
orily and irrevocably denying.') 

But although, immediately after telling us these things, 
the Astronomer Royal for Ireland adds, ' I am beyond all 
things anxious to give you the impression that I am not in- 
dulging in any mere romance,' we may indeed place a great 
deal more reliance on what he says later respecting the 
evidence given by the moon's present rate of rotation. It 
is utterly incredible that the moon, when first formed, no 

1 It has been stated in the Spectator that I believe in the probability 
that all life will be destroyed from off the face of the earth a few years 
hence This was at least news to myself. I have discussed the pro- 
bability that a certain comet will be absorbed by the sun, mentioning 
some one else's suggestion that such destruction might be effected a few 
years from the present time ; but I have also been careful to explain 
that what has already happened in the case of this very comet, shows 
how very small is the chance that the final absorption of the come t 
will iri any way affect the earth's inhabitants, I have scarcely ever 
mentioned such fears except to ridicule them. 



52 MYSTERIES OF TIME AND SPACE, 

matter what theory of her formation we accept, rotated any- 
thing like so slowly as she does at present. It is to all 
intents and purposes certain that then — whenever ' then ' 
was — she rotated in much less than 24 hours. Now she 
requires 27I days for each rotation. There is here evidence 
of an enormous amount of work done by the earth in raising 
and maintaining lunar tides, for by such work alone could 
the moon's rotation rate have been changed to what it 
now is. 

Whether the moon formerly had oceans, as most astro- 
nomers believe, or not, matters little. We see from her 
present aspect that she was once intensely hot, insomuch 
that the greater part of her substance, if not fluid, must have 
been viscous and plastic. In that plastic mass the earth 
raised tidal vibrations, swaying the moon's rotation rate into 
accordance with her period of revolution round the earth. 
In the constancy with which the unjustly called 'inconstant 
moon ' turns ever the same face towards the earth, we re- 
cognise the long-continued action of these tidal vibrations. 
As Dr. Ball well says — ' Those tides have ceased for ages ; 
their work is done ; but they have raised a monument in 
the moon to testify to the tidal sufferings which the moon 
has undergone.' t 

What the earth has done, effectively though slowly, to 
the moon, the moon will do as effectively, though feven 
more slowly ; to the earth. It is this cause of change, of 
the efficiency of which the moon's calm face is ever speaking 
to us, that will produce the lengthening of the day, arid of 
the lunar month, which we have already considered. 

I do not altogether agree with Dr. Ball as to the ftuture • 
of the earth and moon lying beyond the sufficiently distant 
future to which we have already carried our thoughts., He 
points out that besides the lunar there is a solar tid^, and 
that after the former has done its work in bringing the 
earth's rotation period to coincidence with the lunar ntionth, 
the latter still checking the earth's rotation, will causve the 
terrestrial day to exceed in length the lunar month./ He 



THE BIRTH OF THE MOON. 53 

considers that in the case of Mars's internal satellite such a 
change has already been brought about, the satellite re- 
volving around Mars in a period shorter than that of the 
planet's rotation. It appears to me that the two cases are 
not analogous. The mystery of the inner satellite, Dr. Ball 
tells us, ' has never been explained : it is due to the action 
of the solar tides on Mars ; nay, more, we can actually 
foresee that at some incredibly remote future time our earth 
and moon are destined to present the same movements 
which have seemed so anomalous in Mars.' He appears to 
overlook the effects which the outer satellite would tend to 
produce, and also what we notice in the case of our own 
moon. We can readily understand how, with an outer 
moon travelling in longer period, the Martian day would 
have increased in length beyond the time of the inner 
moon's rotation : whereas we see in our own moon clear 
evidence that the solar tide has not the power which Dr. 
Ball here assigns to it — or rather, that whatever effects it 
may exert in that way, are overborne by greater forces 
working in an opposite direction. Ever since the moon's 
rotation-period was brought into agreement (by her earth- 
raised tides) with her period of revolution, she has been 
subject to the sun's influence in still further lengthening her 
period of rotation — this influence being somewhat stronger 
on her than on Mars, despite her smaller globe. Yet during 
the millions of years that this force has been at work, it has 
not in the slightest degree availed to lengthen the rotation 
period beyond the period of revolution. These periods 
were, and remain, absolutely coincident. The reason is 
obvious : the earth has exerted a greater force to prevent 
such an increase of the moon's period of rotation than the 
sun has exerted to produce it. In like manner, we may 
safely conclude that, whenever the moon has wrought the 
terrestrial day into coincidence with the lunar month, she 
will continue thenceforth to maintain that coincidence — 
overruling all the efforts which the sun will make to still 
further lengthen the terrestrial day. 



54 MYSTERIES OF TIME AND SPACE. 

For my own part, however, I believe that long before 
that time arrives, every particle of water will have disap- 
peared from the earth's surface — the seas and oceans being 
withdrawn into the earth's interior as her mass parts with 
its heat. That any living creatures will exist on the earth 
at the remote time to which our thoughts have been carried, 
seems to me altogether improbable. 



55 



BIRTH AND DEATH OF WORLDS. 

To the child in its nurse's arms the room in which his life is 
passed seems the whole world. But as the child grows older 
he finds the room to be but part of a house, the house part 
of a street, the street part of a city, and so forth. Gradually 
he gets larger and larger ideas of the earth which is his 
home. What is true of each individual child, is true of 
the childhood of the human race. In long-past ages men 
judged the narrow tract, island, or valley where their lives 
were passed as the whole world. For that region the 
heavens were made • the sun rejoiced as a giant to run his 
course, that he might illuminate that abode by day • the 
moon was made to be its light by night ; sun, moon, and 
stars to be for signs and for seasons, and for days and years. 
But gradually men widened their conceptions of the world. 
They found the home of their tribe to be but part of the 
region which their race inhabited, this to be but part of a 
larger region. Wider and wider became their knowledge of 
the earth, until they found that what they had regarded as 
the whole world was the merest point on the great globe 
which is the home of the human race. 

But this was little. It may be said that this was nothing. 
Men had found the earth, their home, to be much vaster 
than they had imagined ; but they presently found that in 
another sense it is exceedingly minute. They had judged 
it to be the great fixed centre of the universe, even after 
they had found that it is not a great level tract of land and 
water, but a globe. They now found that it is not fixed, but 



56 MYSTERIES OF TIME AND SPACE. 

in motion ; not the centre of the universe, but a member 
only of a family of globes, travelling like it around the sun, 
and very much less than the larger of these, while compared 
with the sun it is but as a point. 

Even this, however, was far from being all ; it was 
indeed but as a short advance towards the grand conceptions 
of the universe which belong to the science of our day. 
Just as the earth had come to be regarded as but a point, 
almost simultaneously with the discovery that it is a vast 
globe, with a surface of two hundred millions of square 
miles, so the very discovery which assigned to the sun his 
real position in the solar system, showed also that in one 
sense he is but a minute part of the universe. It was rightly 
objected by Tycho Brahe to the Copernican theory 
(though the objection was overruled as observation ad- 
vanced), that if the theory were true, the stars ought to 
change in apparent position as the earth circled on her 
wide orbit round the sun. The objection is sound, the 
stars must change in their apparent position, though in 
Tycho Brahe's time not one star could be shown thus to 
move, and even now there are not ten stars whose motions 
of this sort have been effectively measured. But the Coper- 
nican theory is~sound all the same. It is the vast distance 
of each star that causes it to appear all the year round in the 
same position on the star sphere, though the point from 
which we observe the star circles year by year round the sun 
in a mighty orbit, a hundred and eighty millions of miles in 
diameter. Even in the time of Tycho Brahe, when the 
sun's distance was supposed to be but a few millions of 
miles, it was seen that the only possible explanation of the 
stars' apparent fixity of position involved such vast con- 
ceptions of their distances as to compel us to believe that 
they are suns scarcely less important than our own. 

But as astronomers increased their estimate of the sun's 
distance, and as, observing more. and more carefully the 
stars' positions, they diminished the possible range of the as 
yet undetected apparent motions, men's conceptions of the 



BIRTH AND DEATH OF WORLDS. 57 

grandeur of the material universe increased. With Briarean 
arms science thrust back the stars into the depths of space, 
until the glories of the nocturnal heavens were changed from 
so many thousand points of light to as many suns, many as 
grand as our own, many far grander, some, like Sirius, Vega, 
and Canopus, so much vaster than he is, that by comparison 
with them he seems the merest miniature of a sun. 

But even this, stupendous though it seems, is little com- 
pared with the scene presented when we rightly interpret 
what the telescope reveals respecting the depths of space 
beyond the domain of the visible stars. For each star we 
can see, thousands were made visible by the telescope of 
Galileo, in later times tens of thousands, and in the days of 
the elder Herschel hundreds of thousands. With the best 
telescopes in our own time it is probable that as many as a 
thousand million stars could be seen were every part of the 
celestial sphere examined. 1 A thousand million suns, a 
thousand million repetitions of the glories and the wonders 
which modern science reveals in the central orb of our 
system ! 

But the very progress of our knowledge of the vastness 
of God's domain in space has taught us to recognise the 
incompleteness of our knowledge. When men supposed 
that the orbs they see in the heavens represent the whole 

1 This estimate is greater than that usually adopted. Chacornac 
considers that with the gauging telescopes of the Herschels 20,000,000 
stars could be seen. But if we consider that the 2^-inch telescope 
used by Argelander in his Durchmusterung showed no fewer than 
324,000 stars in the northern heavens, clearly enough for their places to 
be exactly determined, and would certainly show 500,000 in each hemi- 
sphere clearly enough for counting, it is evident that Chacornac's 
estimate of the number which could be seen with the 18-inch mirror of 
the gauging telescope is far too low. Assuredly, if a 2^-inch telescope 
shows more than a hundred times as many stars as the naked eye, it is 
certain that the great gauging telescopes would show more than a hundred 
times as many as Argelander's, or more than a hundred millions, and 
as certainly there are many telescopes in the present day which will 
show more than ten times as many as the Herschelian telescopes. 



58 MYSTERIES OF TIME AND SPACE. 

glory of the universe, they were content with what they saw, 
and supposed they knew all. Now that we know how little 
men knew in past ages, we no longer believe that we can 
now extend our range of view over more than the manifest 
portion of the real universe. 

It would have been natural, but strangely enough it did 
not happen, that men's ideas of the duration of time should 
have grown, pari passu, with their ideas of the vastness 
of space. So long as they had recognised as the entire 
universe a small rounded region of the earth's surface, 
arched over by the dome of the sky, in which certain bright 
objects, sun and moon and stars, seemed to travel, a very 
moderate time-interval seemed long enough for the entire 
duration of the universe as well in the future as in the past. 
But with every increase of men's estimate of the extent 
of the universe came an increase in their estimate of the 
duration of the time during which the natural universe had 
existed in the past and would continue to exist in the 
future. Only, in one case, proof positive forced men to 
enlarge their conceptions ; in the other, abstract reasoning 
alone was at first available, and even later the proof was 
not so absolutely convincing (at least to the weaker order of 
minds) as that which showed the vast extension of the 
universe in space. 

Applying this principle, let us see what we must think 
of time past and to come. Our views of the vastness of 
space have ranged from the comparatively minute to the 
inconceivably vast, and beyond that we look into the depths 
of the immeasurable, the infinite. So it must be with time. 
From the few thousands of years which seemed to suffice 
for the small universe of the ancients, our thoughts have 
risen to the recognition of millions on millions, nay, of 
millions of millions of years, and they should pass beyond 
these, if we rightly apprehended the teachings of science, to 
the contemplation of absolute infinity of time in the past as 
in the future. 

But as our ideas as well of the vastness of space and the 



BIRTH AND DEATH OF WORLDS. 59 

duration of time have grown, so also should our estimate 
grow of the range of the operation of what we call law. In 
past days men might perhaps reasonably fear that if they 
extended their belief in development beyond what they 
could unmistakably recognise in the growth of animal or 
plant, they would see evolution extending its domain 
throughout the entire universe, as they knew it. But as 
their estimate of the universe extended, this fond fear 
vanished. From a plant to a forest, from a forest to a flora, 
they felt they could safely carry their survey of the vegetable 
world. From an animal to a species, from a species to a 
fauna, they could safely trace the evolution of animals. 
Even more widely might their sway be extended. Until at 
length men saw that they might recognise evolution through- 
out the entire domain of the universe as they knew it, and 
yet be no nearer the First Cause, than of old they were in 
believing that the tiniest plant or animal grows according to 
natural laws. 

It is in this spirit, in the full belief that so far as man 
can extend his survey he can trace the existence of law, 
without even coming near the First Cause (for what relation 
save that of absolute nothingness can the finite bear to the 
infinite?), that we approach the consideration of the de- 
velopment of a small part of the universe — the merest atom 
in space — our solar system, the sun, with his family of 
dependent worlds. 

Science considers it now as to all intents and purposes 
established that this universe 

in tracts of fluent heat began 
Till toward the centres set the starry tides 
That eddied into suns, that wheeling cast 
The planets— then the monster — then the man. 

It may be doubtful in what precise manner the process took 
place, but that each orb in our solar system, for instance, 
was once a mass of vaporous matter widely extended in 
space, no modern student of astronomy doubts. 

The first stage, then, in which we recognise a world or 



60 MYSTERIES OF TIME AND SPACE. 

planet like our own is that of a mass of glowing vapour. 
And when I say vapour, I do not mean one kind of vapour 
only, but that every substance which now exists in this 
world in the solid or liquid state was once, through intensity 
of heat, in the form of vapour. 

Now let us consider what are the characteristics which 
we might expect to recognise in a world in such a condition 
as this. First, it is clear that, as only a very intense heat 
could convert into the vaporous form the substances which 
we know on the actual earth as solids, an orb in this primary 
state of a world's existence would be so hot that we might 
fairly expect its heat to be felt even at such distances as 
separate planet from planet, or the planets from the sun. 
Nor can we doubt that when there was such intense heat 
there could be also most brilliant light. Then again, the 
vaporisation of such a world as ours would necessarily be 
accompanied by a vast expansion of its size, and therefore 
by a great diminution of its mean density, so that we 
cannot doubt that an orb in the first or vaporous state of 
its existence would have a smaller mean density than later, 
when its materials had become in great part solid or liquid. 

Such, then, would be the characteristics of the first stage 
of a world's life, and by such characteristics may we know 
a world in this stage of its career, if any such now exist : 
intense heat, heat that can be felt and measured, brilliant 
lustre, and small mean density. The reader will see pre- 
sently why we note these features. 

And now let us consider the next stage through which 
such a world as we have considered would have to pass. 

After a time the radiation of heat from the mass of 
glowing vaporous matter would result in so great a diminu- 
tion of the heat originally pervading the mass, that a large 
portion of the matter which had been vaporous would 
become first liquid, then solid. The substances, for instance, 
which now form the solid crust of the earth would thus 
become solid, and so would a large proportion of the 
material now existing in that form. Others of these, though 



BIRTH AND DEATH OF WORLDS, 61 

not becoming solid, would be changed to the liquid form. 
But there is one substance existing in large quantities now 
in the liquid form, which assuredly could not become liquid 
for a very long time after the earth first began to have a 
partly liquid and partly solid nucleus. We refer to water, 
which covers now three-quarters of the earth's vast surface 
of 200 millions of square miles. When the earth's first 
formed surface was still glowing with its primeval fires, it 
is manifest that water could no more have rested on that 
surface than it can now rest on a surface of redhot iron. 
We do not say that every part of it would of necessity be 
converted into steam or vapour. We do not even say that 
no water would be in contact with the fiery hot surface. 
We say simply that it could not rest against that surface. 
Where the pressure is so enormous as at the bottom of 
our present seas (for instance), water may remain in contact 
with a surface many times hotter than boiling water and 
yet not be converted wholly into steam. Yet it could not 
possibly rest under those conditions. Vast quantities of 
steam would rush upwards through the superincumbent 
water (which would be partly vaporised by the intense 
heat of the uprising vapour) passing with enormous turmoil 
and uproar into the higher air. And unquestionably, if 
the whole surface of the earth were at such a heat as that 
of glowing iron, the greater part of the waters now forming 
our seas and oceans would be always in the form of vapour, 
while the portion which would remain liquid would be 
constantly interchanging that condition for the vaporous 
one, the supply remaining tolerably constant, because, as 
fast as water was turned into steam and flung upwards, vast 
quantities of water formed by the condensation of the 
immense clouds constantly brooding over the fiery earth 
would rush down in torrential showers to take its place. 

Now let us consider what would be the aspect of a world 
in this second stage of orb-life, which we may call the fiery 
stage. 

In the first place, it is tolerably clear that, seen from 



62 MYSTERIES OF TIME AND SPACE. 

without, such a world would present only the outside of its 
cloud envelope to view. In proportion as the quantity of 
steam forming a part of the complex atmosphere of the 
earth was enormous, so also would the cloud masses en- 
wrapping the earth be vast and the layers formed by them 
be deep, and impenetrable either from without or from 
within. Above our earth, even in its present condition, 
three distinct layers of cloud frequently form over one and 
the same region, besides other but less distinct intermediate 
layers. There are on the outside the light feathery clouds 
called cirrus, really consisting of minute particles of ice. 
Next to these, but far below them — usually perhaps at least 
nine or ten miles below — are the woolpack clouds called 
cumulus, showing where great quantities of aqueous vapour 
have been changed into the form of visible cloud. Below 
these again, and often clinging to the very surface of the 
earth, are the clouds in which actual precipitation of rain is 
taking place, the so-called nimbus clouds. Between these 
layers may be often seen the cirro stratus and the cumulo 
stratus. Now if this is the case with the present clouds, 
formed under far more stable conditions and from a com- 
paratively insignificant quantity of the vapour of water, how 
many and how complex must have been the cloud layers, 
how deep and dense must they severally have been, when 
millions of millions of tons of water were present in the 
atmosphere in the form of steam ! 

We might certainly expect, then, to find a world in that 
second or fiery stage of world life, presenting a surface 
manifestly formed in great part, if not wholly, of cloud 
masses. We might also certainly expect to find the surface 
so seen undergoing frequent, and sometimes very rapid, nay, 
tremendous changes. Thirdly, since the apparent size of 
such a globe would be that corresponding to the outermost 
surface of its outermost cloud layer, it would appear much 
larger than it really was, and would be judged by observers 
unaware of its real condition to be of much smaller mean 
density than was actually the case. 



BIRTH AND DEATH OF WORLDS. 63 

Such would be the characteristics of an orb in the 
second or fiery stage of a world's life : a cloud surface, 
indications of intense activity, and relatively small mean 
density. 

Then would come the third or middle stage of planetary 
life, for which the two preceding stages had been merely 
preparatory. The surface would become at last so cool that 
the waters which had hitherto been suspended in mid air, 
in the form either of steam or of cloud masses, would be 
able to rest on the surface as seas and oceans. Dry land 
would appear, life would become possible on the transformed 
world. Whether, under the conditions existing just at this 
stage of the world's existence, life could in any way come 
spontaneously into existence, may be a subject for specula- 
tion or perhaps hereafter even fcr theorising. Science, at 
any rate, knows nothing now of the possibility of spontaneous 
generation, so that the peopling of a world with life is as 
great a mystery now as it has ever been. Men of science 
may amuse themselves by speaking of life being brought to 
the earth by the arrival of a meteor, in reality a fragment of 
some once peopled world, which has been destroyed by con- 
flict with another or by internal disturbance. But this is 
more a scientific jest than a grave reality. Astronomy 
knows nothing of worlds coming into conflict. On the 
contrary, the laws of motion assure us that if anything is so 
unlikely that it may be regarded as absolutely impossible, it 
is the encounter of two orbs in mid space ; nor have we any 
reason to suppose that a planet can be rent into fragments 
by internal convulsions. If we had, we have not the slightest 
reason for supposing that orbs thus unfortunate would be 
more likely to be inhabited than their more lucky fellow- 
worlds. If these were inhabited already, we gain nothing 
by bringing to them the fragments of other worlds which 
have exploded ; and if they were not inhabited, while the 
burst or shattered worlds were, we are called on to imagine 
(for no one can believe) the absurdity that only inhabited 
worlds are liable to destruction, for the benefit of those 



64 MYSTERIES OF TIME AND SPACE. 

which are without inhabitants. To which absurdity this, 
additional one is superadded, that the seeds of life would 
survive the destruction of their planet home, and the journey- 
ing through millions on millions of years (rather millions of 
millions) which science assures us they would have to make 
through the cold of interstellar space, before they would fall 
on any other world. And all these absurdities to no purpose, 
so far as the origin of life is concerned, for they take us back 
but a step, which brings us in reality no nearer to the begin- 
ning of all life. 

But we are not concerned to inquire here how life was 
introduced upon this earth or any other world. We know 
that in some way life began after the earth became fit to be 
the abode of life ; and again we need not inquire what would 
be the appearance of a planet in the life-bearing stage, seen 
from a distance, as we see Mars or Venus. For we require 
no extraneous evidence to show that an orb passes through 
the life-bearing stage, seeing that we know from our own 
earth that this is the case. 

We may pass on, then, at once to consider the later stages 
of planetary existence. 

Here we no longer have the assistance which we have 
in the case of past stages, from a study of the earth's actual 
condition. It is easier to infer from the earth's present state 
what her past has been, than to predict what her future will 
be. For the traces of past changes remain, the effects of 
such changes having as it were accumulated, but the signs 
of changes yet to take place are not so obvious, because the 
stages of a world's life last very long, and the amount of 
change which can be actually observed in the lifetime of a 
man, or even of a race, is almost imperceptible. 

But there is one change which, though in such periods 
as we have just named it hardly manifests any effect at all, 
must yet be all the time in progress. It was shown long 
since by Sir Isaac Newton (and though the method of his 
reasoning is rather out of date, the principles on which it 
rested are sound enough) that the seas and oceans of this 



BIRTH AND DEATH OF WORLDS. 65 

earth must all the time be diminishing, though so slowly 
that in many generations no visible change of level can be 
perceived. 

The theory which with Newton was little more than a 
speculation has in our own time been placed on a secure 
footing. Advanced almost simultaneously, but independ- 
ently, by Saemann in Germany, by Stanislas Meunier in 
France, by Dr. Sterry Hunt in America, and by Frankland 
in 'this country, it may now be regarded as established on a 
basis of strong probability, to say the least. 

In the long periods of time which belong to the earth's 
lifetime, processes which in the lifetime of a man or in the 
history of a nation produce scarcely any discernible effect 
may so act as entirely to modify the aspect of a world. It 
becomes more and more clear, as we study the earth's 
history, that it must be measured, not by thousands of years 
or by tens of thousands, but at the very least by millions of 
years. Now, let us imagine that the rate at which the water 
in our seas and oceans is withdrawn into the interior is so 
slow that in a single year the sea-level is reduced by an 
amount equal to about the thickness of a sheet of paper. 
Then in a hundred years the depth of the sea would be 
diminished only a single inch. At this rate, in about 
6,400,000 years the sea-level would be reduced a full mile, 
and in 60 millions of years every trace of water would have 
disappeared from the surface of the earth. 

Here we must not fall into the mistake of describing the 
water as withdrawn into cavities formed within the earth as 
she cools. It does not seem to be so generally recognised 
as it should be that below a certain depth there can no 
more be cavities within a planet's globe than there can be 
holes in the masses of water forming the seas and oceans of 
the planet. The pressures which exist deep down below 
the surface, at a depth for instance of 20 miles, are so great 
that the hardest substances at such pressures become plastic 
and behave like fluids. In Tresca's experiments, for 
instance, on the behaviour of steel under great pressure, it 

F 



66 MYSTERIES OF TIME AND SPACE. 

was found that the metal behaved like a viscous fluid ; yet 
the pressures Tresca could apply were as nothing com- 
pared with those which exist at a depth of twenty miles 
below the surface of the earth. Thus, at such depths 
there could not be cavities of measurable dimensions. 
Such cavities, however, as exist in porous bodies, could 
and would exist in the crust and throughout the frame of a 
cooled planet, and into such cavities the waters of the seas 
and oceans would unquestionably be withdrawn by the 
action of capillary attraction. 

Within periods of time such as correspond to the lifetime 
of a planet, the water on our earth will be perceptibly 
diminished and eventually will disappear altogether. 

In an old planet, then — that is, a planet which has 
passed from beyond the life-bearing stage in which our earth 
now is — the water surface is much less than in the time of 
mid-life. We should recognise a planet in this its period of 
decrepitude, by the smaller extent of its water surface. If 
we supposed all planets to be fashioned on the same general 
model as our earth, we can by studying deep-sea soundings 
ascertain even the general shape and appearance of the 
diminished seas which would then remain. We find, when 
we have done this, that the smaller seas would have such 
shapes as we recognise in the Baltic — there would be many 
seas lying, so to speak, within continents, but connected by 
comparatively narrow inlets with the oceans, which would 
still remain, though much diminished in extent. If there is 
a world within the range of telescopic vision which is in 
this stage — the fourth — having passed through the glowing 
vaporous stage, the fiery stage, and the life-bearing stage, 
we might expect to recognise it thus by its diminished sea- 
surface, and by the shape of its smaller seas. 

The last stage — when no trace of life could remain — 
would be attained when every trace of water had dis- 
appeared from the planet's surface. We should recognise a 
planet in this stage by the utter absence from its disc of 
every trace of cloud, It is probable that with the dis* 



BIRTH AND DEATH OF WORLDS. 67 

appearance of all the water the atmosphere of a planet 
would be greatly reduced in density and extent. But even 
if the air remained, it would be a perfectly dry atmosphere. 
No clouds could form, no rains fall, no changes such as we 
regard as meteorological could possibly take place. The 
planet's surface would present a dead, arid waste. 

Such, then, are the five stages of a world's life — the 
glowing vaporous stage, or infancy, the stage of fiery youth, 
the life-bearing middle stage, the time of decrepitude, and 
finally the stage of death. 

But if every orb passes through these stages, we might 
fairly expect to find examples of every one of them, or at 
any rate of others besides that stage of middle life in which 
our own earth is, among the numerous orbs which the 
telescope enables us to study. For we can hardly imagine 
(on a priori considerations) that all the orbs in each system 
would be in the same stage of a world's existence. Even if 
we supposed, which we have not the slightest grounds for 
doing, that they had all started into existence at one and 
the same epoch, we should yet expect to find that the stages 
of life in bodies which differ so much in size would differ 
considerably in length. Were this so, if they all began 
their career as worlds at the same time, they would attain 
each stage of their life at very different times. This 
moment, for instance, which belongs to the mid-life of the 
earth, might correspond to the fiery youth or even to the 
vaporous infancy of another world, and to the old age or 
even the final or deathlike stage of another. 

But before looking around among the orbs of space, 
may we not adopt some law for our guidance by which we 
may form some idea as to which worlds are likely to be 
young, and which to be old ? 

If we consider that the stages of a planet's life depend 
on the degree of temperature to which the substance of the 
planet is raised, we shall see that such a law as we have 
mentioned may be deduced from a consideration of the 
laws according to which heated bodies cool. The planets, 

f 2 



68 MYSTERIES OF TIME AND SPACE. 

we may assume, are made of the same materials, though of 
course such materials may be differently proportioned in 
different planets. Thus we need not consider the different 
rates at which bodies of different materials cool. We know, 
however, that the planets differ greatly in size. Now, bodies 
of the same material, but of different size, take unequal 
times in cooling. If we heat two globes of iron, one an 
inch in diameter, the other two inches in diameter, to a red 
heat, and set them to cool, we find that the larger takes a 
much longer time in cooling than the smaller. When the 
former has lost most of its former ruddy lustre, the larger 
still remains aglow with heat. When the smaller has at 
length become so cool that it can be readily handled, it 
would be unpleasant to try the same experiment with the 
larger. When we ask why this is, we readily find an answer. 
The amount of heat that a body contains is proportional to 
the mass of the body. In the case of two globes of iron, 
for instance, at the same temperature, one globe being one 
inch, the other two inches in diameter, the quantity of heat 
which the larger globe has to part with exceeds that 
possessed by the smaller globe, in the same degree that the 
larger globe exceeds the smaller in mass — that is, as eight 
(the cube of two) exceeds one. But the heat can only pass 
away at the surface, and the surface of the larger globe is 
not eight times, only four times, as large as that of the smaller. 
Since, then, when both are at the same temperature as at first, 
the larger has eight times as much heat to part with as the 
smaller, and since it parts with that heat only four times as 
fast as the smaller (or at only half the rate at which it would 
have to part with its heat to cool as fast), it follows that 
the heat of the larger will last twice as long as that of the 
smaller. Or, to speak more exactly, each stage of the 
cooling of the larger lasts twice as long as the corre- 
sponding stage in the cooling of the smaller. This is 
exactly the degree in which the diameter of the larger 
exceeds that of the smaller. And a little consideration will 
show that in this degree always (that is, in the proportion of 



BIRTH AND DEATH OF WORLDS. 69 

the diameters) the cooling of the larger of two globes will 
last longer than that of the smaller. 1 For instance, suppose 
the larger three inches and the smaller two inches in 
diameter, then the mass of the larger exceeds that of the 
smaller twenty-seven times, while the surface of the larger 
exceeds that of the smaller only nine times. Since, then a 
supply of heat twenty-seven times as great is parted with, 
not twenty-seven times, but only nine times as fast, it 
follows that the duration of the supply will be greater in 
the same degree that twenty-seven exceeds nine, or three 
times, and this is the degree in which the diameter of the 
larger globe exceeds that of the smaller. 

Now, it would be by no means safe to assume that the 
duration of each stage of a planet's life will be proportional 
to the planet's present diameter. For the planets do not 
resemble the globes of our reasoning in being of the same 
mean density. We might be nearer the mark, perhaps, in 
supposing that the duration of a planet's life is proportional 
to the cube root of the number representing the planet's 
mass, for then we should be taking the diameters which the 
planets would have if they were all at the same density. But 
even this would be an unsafe assumption, for it is by no 
means likely that two planets of very different mass, even 
though of similar constitution, would be of the same mean 
density when passing through the same stages of planetary 
existence. But by this second method we should probably 
so far approach the truth as to obtain a fair idea of the rela- 
tion between the lifetimes of two planets differing in size. 

Thus, we could not safely say that because the diameter 
of Jupiter is ten times that of the earth, therefore each stage 
of the lifetime of Jupiter exceeds tenfold the corresponding 

1 Thus the volumes are as the cubes or third powers of the diameter, 
while the surfaces are as the squares ; so that, since the quantities of 
heat are as the volumes, and the ratio of parting with the heat is as the 
surfaces, it follows that the durations of the cooling must be as the cubes 
divided by the squares (these divisions relate only to the numbers repre- 
senting cubes and squares) or as the diameters directly. 



70 MYSTERIES OF TIME AND SPACE. 

stage of our earth's life. Nor can we say, though this might 
be nearer the mark, that because Jupiter has a mass very 
nearly equal to seven-times seven-times seven-times the mass 
of the earth, therefore each stage of Jupiter's life is seven- 
times as long as the same stage of the lifetime of the earth. 
But though neither of these results may be even near the 
truth, the second probably gives a fair idea of the enormous 
difference between the duration of each stage through which 
our earth has passed, and the duration of the same stage in 
the lifetime of Jupiter, whether he has already passed through 
such stage, or still has to do so. 

Now take the numbers which such a comparison intro- 
duces, in the case first of an orb like Jupiter, very much 
more massive than the earth, and then in that of an orb of 
very much smaller mass, like our moon. 

It has been shown that had past geological changes in 
the earth taken place at the same rate as those which are 
now in progress, one hundred millions of years at the very 
least would have been required to produce those effects which 
have actually been produced, we find, since the earth's surface 
was fit to be the abode of life. But recently it has been 
pointed out, correctly in all probability, that under the 
greater tide-raising power of the moon in past ages, these 
changes would have taken place more rapidly. As, however, 
certainly ten millions of years, and probably a much longer 
time, must have elapsed since the moon was at that favour- 
able distance for raising tides, we are by no means enabled, 
as some well-meaning but mistaken persons have imagined, 
to reduce the life-bearing stage of the earth from a duration 
of a hundred millions of years to a minute fraction of such 
a period. The short life, but exceedingly lively one, which 
they desire to see established by geological or astronomical 
reasoning, never can be demonstrated. At the very least, we 
must assign ten millions of years to the life-bearing stage of 
the earth's existence. If now we multiply this period by seven 
for Jupiter, we get a period sixty millions of years longer ! 

But take the stage preceding that of life on the earth. 



^— ■ 



BIRTH AND DEATH OF WORLDS. 71 

From the researches of Bischoff into the cooling of masses 
of heated rock, it seems to follow that a period of more than 
three hundred millions of years must have been required 
for the cooling of the earth from a temperature of 2,000 
degrees Centigrade to one of 200 degrees, a cooling which 
has certainly taken place. Suppose, however, that these 
experiments, or the calculations based on them, were 
vitiated by some error so considerable as to increase the 
real duration of the fiery stage of our earth's history more 
than tenfold, the real duration of that period being only 30 
millions of years. Multiply this in turn by 7, and we get a 
period of 210 millions of years, or 180 millions of years 
longer. 

We ought next to consider the vaporous stage ; but the 
evidence on which to form an opinion as to the duration of 
this stage of a planet's history is too slight to be the basis 
of actual calculation. Here, as Tyndall has well remarked, 
'conjecture must entirely cease.' 

But, by considering only two stages — the fiery stage and 
the life-bearing, or rather that portion of the life-bearing 
stage through which the earth has hitherto passed — we find 
the two monstrous time-differences — 180 millions and 60 
millions of years, or 240 millions of years in all. They 
mean that, if our assumption as to the effect of Jupiter's 
superior mass is correct, then, supposing Jupiter and the 
earth to have started into existence as distinct orbs at the 
same or nearly the same time, 240 millions of years must 
elapse before Jupiter will reach the stage of planetary life 
through which our earth is now passing. Whether the 
assumption be correct or not, the time-differences between 
the stages of Jupiter's life and the earth's are of this order. 
They must be measured by tens of millions if not by 
hundreds of millions of years. 

We must note, however, that the 240 millions of years 
correspond with only a seventh part of that time in the 
earth's history : so that we may say that, if our assumptions 
are correct, Jupiter would now be in the state in which our 



72 MYSTERIES OF TIME AND SPACE. 

earth was 34 millions of years ago, or nearer the beginning 
than the end of the fiery stage. 

Take next a much smaller orb than the earth — the 
moon, for example, with a mass equal but to the 81st part 
of hers. Here we may compare the actual masses and 
surfaces, knowing that when the earth has parted with all 
her internal heat her mean density will not exceed very 
greatly that which she has now. (It would strengthen our 
argument to suppose that she would then be very much 
denser, or, which amounts to the same thing, that the moon, 
when in the same stage of planetary life as the earth, was 
very much less dense than at present.) 

The moon's mass is but the 81st part of the earth's, her 
surface being less than the earth's about as 2 is less than 27. 
Since the earth, when at any given stage of her career as a 
planet, had 81 times as much heat as the moon, and since 
she had a surface not 81 times but only 13^ times greater 
than the moon's, each past stage of her cooling has lasted, 
and each of those to come will last longer than the corre- 
sponding stage of the moon's cooling, in the same degree 
that 81 exceeds 13 J, or about 6 times. Taking, then, the 
periods we adopted when comparing the earth and Jupiter 
— viz. to millions of years for the life -bearing stage and 30 
millions for the fiery stage — we find that these periods of 
the moon's history would have lasted if millions of years and 
5 millions of years respectively. Or, supposing the moon and 
earth to have begun the fiery stages of their respective lives 
at the same time, the moon would have attained the earth's 
present condition after 6§ millions of years, or (taking this 
period from 40 millions of years) 33 J millions of years ago. 
Note further that during these 33 millions of years the 
moon would have been cooling, not at the earth's rate but at 
her own much more rapid rate, or six times as fast. They 
would correspond, therefore, with six times 33 J millions of 
years, or 200,000,000 years in the history of the earth. 

We see, then, that, while we might expect to find 
Jupiter (as we have seen) in an earlier part of the fiery stage 



BIRTH AND DEATH OF WORLDS. 73 

of planetary life, we might reasonably expect to find the 
moon very far advanced in planetary decrepitude. More- 
over, we see that, as Jupiter is the largest of the planets 
and the moon the smallest, 1 we may fairly expect to find 
among the various members of the solar system illustrations 
of all the stages of a planet's existence^ from that through 
which Jupiter is passing (presumably the fiery stage) to that 
through which our moon is passing (presumably either the 
stage of extreme decrepitude or of death). 

But is there one orb in the solar system which might be 
expected to illustrate the first stage of all, the glowing 
vaporous stage which may be regarded as corresponding 
with the infancy of planetary existence ? 

There is one orb so much larger than any other that we 
might expect him to be far younger even than Jupiter. 
That orb is the sun, with a diameter exceeding Jupiter's in 
about the same degree that Jupiter's exceeds that of the 
earth, with his volume exceeding the earth's a million and a 
quarter times, with his mighty mass exceeding hers 326,000 
times, and Jupiter's not much less than a thousand times. 
If we applied to him the same reasoning that we have 
applied to Jupiter, we should expect to find each stage of 
his tremendous life-existence ten times longer than the 
corresponding stage in the existence of Jupiter. Setting 40 
millions of years as the time since the earth began the fiery 
stage of her career, and 280 millions of years as the time 
required by Jupiter to complete the changes which the earth 
has passed through in those 40 millions of years, we should 

1 We except, of course, the asteroids, each of which is far smaller 
than the moon. The satellites of other planets need not be excepted ; 
for not one of them can, like the moon, be regarded as a planet. They 
really are dependent bodies, utterly small and insignificant compared 
with the orb round which they travel. With the moon it is different, 
she may justly be described as our earth's companion planet. A 
modern astronomer has described her as a planet travelling round the 
sun and largely perturbed by the attractions of a neighbouring planet — 
the earth. 



74 MYSTERIES OF TIME AND SPACE. 

find for the sun no less than 2,800 millions of years. That 
is, taking no account of the vaporous stage, in which never- 
theless it may be presumed that he still is, it would follow 
that 2,800 millions of years, less 40 millions, would have to 
pass before the sun's mass will have cooled to the same 
degree as the earth's at this present time. We may, then, 
certainly expect to find the sun in the first or glowing 
vaporous stage of orb life, with at least as much reason as 
we found in support of the belief that Jupiter is in the 
second. 

Now let us briefly consider whether the characteristic 
features which we have seen to belong to the five stages of 
an orb's life are really shown by members of the solar 
system which, judging from their dimensions, we might 
expect to find in those stages of life respectively. 

Turn first to the sun. We need no telescopic scrutiny 
to assure us that he at least presents two of the character- 
istics assigned to an orb in the glowing vaporous state. He 
unquestionably pours forth enormous quantities of heat, 
he assuredly glows with an intense lustre. The glory of the 
sun and his fiery heat assure us as with the clearest words 
that he is in the infancy of his career as an orb in space. 
When, however, we examine his structure with the telescope, 
and, taking the aid of that wondrous instrument of research, 
the spectroscope, ascertain the very materials of which his 
glowing mass is formed and determine their condition, we 
have even clearer evidence. His lustre and his heat, how- 
ever accordant with what analogy would have led us to 
expect, might be otherwise explained than by assuming that 
he is simply an orb like our earth, but very much larger and 
in a much earlier stage of existence. He might be made of 
very different materials, and his lustre and his heat might be 
produced in a way entirely differing from an)' known way by 
which on this earth heat and light are generated. But we 
know him, from closer scientific scrutiny, to be made of 
the same materials as the earth, we know those materials to 
be in the vaporous condition which only intense heat will 



BIRTH AND DEATH OF WORLDS. 75 

produce, and we see that the whole of the sun's glowing 
mass is disturbed in such a way as to indicate that the 
greater part of that mass can be in no other than the 
vaporous state. So also, when we determine the mean 
density of the sun, we find that, instead of being far more 
compressed than the earth, as we might expect from his 
enormous mass were he in the same condition, he has a 
mean density only about one-fourth of hers. 

Then outside the visible sun we see the mighty flames 
which leap over his surface to a height of thirty, forty, fifty, 
sometimes even eighty or a hundred thousand miles, great 
masses of glowing gas shot forth from his interior with 
velocities so great that compared with them all forms of 
motion with which we are acquainted on earth seem abso- 
lutely as rest. Outside these again are the mystic streamers 
of the corona, extending to distances of two hundred, three 
hundred, sometimes five hundred thousand, or even a 
million miles. All these features, the wonders of which 
might well occupy our thoughts during a longer time than 
all the rest that we have thus far considered, show that the 
sun is in reality an orb in the first stage of world life, the 
glowing vaporous stage, when the whole frame of an orb is 
instinct with fiery heat and aglow with intense lustre. 

Turn next to the planet which we have regarded as 
likely to illustrate the second or fiery stage of a world's 
life — the planet Jupiter. Or consider, not Jupiter alone, but 
the brother giants, Jupiter and Saturn. 

When we examine these planets with the telescope we 
find that every feature of their disc corresponds with the 
theory that the real surfaces of both these planets are hidden 
beneath deep cloud masses. We find abundant traces of 
intense disturbance affecting these cloud masses. And 
further, when we consider the mean density either of Jupiter 
or of Saturn, we find the most decisive characteristic 
of all — the small mean density, which we expect to re- 
cognise in a planet in the fiery stage, but which certainly 
would not be shown by the giant masses of Jupiter and 



76 MYSTERIES OF TIME AND SPACE. 

Saturn, unless there were some powerful forces at work 
counteracting the tremendous attractive influence of the 
matter composing their tremendous globes. Were Saturn 
or Jupiter in the same condition as the earth, as well as 
made (which we may well believe "to be the case) of the 
same materials, we know that the tremendous pressure to 
which their substance would everywhere be exposed would 
result in producing a density far greater than that of the 
earth with her much smaller mass and relatively much 
smaller attractive forces. But so far is this from being the 
case, that the density of Jupiter is but one-fourth, that of 
Saturn little more than, one-seventh, the mean density of 
our earth. 

In the case of the giant planets, then, as in that of the 
still vaster orb of the sun, we find what theory had led us 
to anticipate. We find that these orbs coming next in order, 
in point of size, to the sun, present all the appearance of 
being in the next stage of orb life. Just as he presents all 
the characteristics which we expect to find in orbs in the 
first or glowing vaporous stage of existence, so do these 
orbs present all the characteristics of the second or fiery 
stage. 

Passing over the third or middle stage, in which we 
know our earth to be, let us seek for an illustration of the 
fourth stage, or the stage of planetary decrepitude. We have 
seen reason to believe that our moon, so much less than the 
earth in size and mass, is probably either in a state of ad- 
vanced decrepitude, or actually dead. If there is any orb 
between the earth and the moon in size and mass much 
less than the earth and much larger than the moon, we 
might fairly expect to find that orb in the middle state be- 
tween the earth and the moon, not so old as the moon, but 
still a great deal older than the earth. 

The planet Mars is such an orb. The moon has a mass 
equal to the 8ist part of the earth ; Mars has a mass equal 
to nearly one-ninth of the earth's, or nearly nine times as 
great as the moon's. So that, precisely as the sun exceeds 



BIRTH AND DEATH OF WORLDS. 77 

Jupiter in mass in about the same degree that Jupiter ex- 
ceeds the earth, so Mars exceeds the moon in mass in about 
the same degree that the earth exceeds Mars. We could 
not therefore have, on a priori grounds, a more convenient 
middle term than Mars, for the stage midway between those 
through which the earth and the moon are respectively 
passing. 

Now, when we make comparison between the earth and 
Mars in that feature which we have regarded as charac- 
teristic of advanced age, what do we find ? On the earth 
the water surface is equal to about three-fourths of the 
entire surface of the globe ; in Mars the surface occupied 
by water is but about one-half (rather less, according to the 
observations of some astronomers). Again, when we examine 
the seas of Mars in respect to their shape, we find precisely 
the feature which we were led to expect in a planet which 
had passed so far beyond the stage in which the earth now 
is that the seas had become greatly reduced in extent. We 
find seas like the Baltic and the Mediterranean as charac- 
teristic in the case of Mars as they are unusual in the case 
of the earth. Nine of the seas on Mars have this peculiar 
shape, which Stanislas Meunier, following myself, has de- 
scribed as bottle-necked. 

So far as we can judge, in fine, all the features of Mars, 
like those of the sun and Jupiter, correspond with the antici- 
pations which we should have based on the planet's mass. 

Lastly, we turn to the moon. This planet, we have 
seen, should be in the state which the earth will reach 200 
millions of years hence, if our assumptions as to the dura- 
tion of the various stages of the earth's existence are correct. l 

1 It is worth noticing that in reality it matters very little whether 
these assumptions are correct or not, so far as our ideas respecting the 
stage of life reached by the several planets are concerned. If we have 
Dver-estimated the duration of the various stages of the earth's existence, 
we have correspondingly under- estimated the rate at which the changes 
which characterise these various stages proceed, so that if we have in 
consequence over-estimated the difference of the durations of the stages 



78 MYSTERIES OF TIME AND SPACE. 

As we have said, it might fairly be expected that at that re- 
mote date the earth will be dead, in the sense of being unfit 
any longer to support life. The moon, then, if our theory 
be correct, might be expected to be a dead world. Now, 
what do we find ? So far as the disappearance of seas and 
oceans is concerned, the moon certainly presents every sign 
of death. There are astronomers, indeed, who consider that 
there are traces of moisture on the moon, and some few ob- 
servers (who certainly are not astronomers) who consider 
that they can trace signs of water-surfaces — small pools and 
so forth — on the moon. But, as a matter of fact, all astro- 
nomers of repute agree that the surface of the moon is 
absolutely arid — dry, desolate, and dead. Again, so far has 
the moon progressed towards the final stage of planetary 
existence, even if she have not absolutely reached it, that 
no trace whatever of an atmosphere can be recognised on 
her surface. As she passes over the stars, she obliterates 
them from our view, with a suddenness which shows that 
either she has no atmosphere or an atmosphere so tenuous 
as to have no appreciable effect in softening the edge of the 
lunar disc, so as to diminish in any perceptible degree the 
suddenness with which stars are obliterated from our view. 
There is no trace of any twilight on the moon. The shadows 
of mountains are black and sharply defined, instead of pre- 
senting those half-tints which we know to exist in the 
shadows of our own mountains. When, standing on some 
mountain summit at sunrise or sunset, we look into the 
shadowed valleys, we do not look into absolute blackness ; 
and if we ask where the light comes from which partially 

of these planets' lives, the changes taking place during such over-esti- 
mated period will not be greater than those really due to the difference 
correctly estimated, for they will take place at a correspondingly slower 
rate. And vice versa, if we have under-estimated the difference of the 
durations of the stages of other planets' lives, the changes taking place 
during such under-estimated periods will not be less than those really 
due to the differences correctly estimated, for they will take place at a 
correspondingly quicker rate* 



BIRTH AND DEATH OF WORLDS. 79 

illuminates those valleys, we find our answer at once. We 
know what we should recognise if we walked into those 
valleys. We should find light reaching us from the sky. 
The valleys could not be in black shadow unless the skies 
were also black. Seeing, then, that the shadows of the 
lunar mountains are absolutely black, we learn as certainly 
as we should if we could actually have visited the moon 
and walked into those shadowed regions, that the sky seen 
overhead there would be absolutely black, with the stars 
shining as brilliantly there in broad daylight as they do 
with us on the clearest and darkest night. Now, our sky 
is in reality illuminated air — air which, receiving the solar 
rays, refracts and reflects it towards the earth, so that to 
say the sky is black as seen from the moon is equivalent to 
saying that either there is no air, or the air is so thin that it 
reflects no perceptible portion of the solar rays. 

Waterless, then, and airless, the moon must be regarded 
as dead. She tells the same story as the sun, the giant 
planets, our earth, and Mars. She is in the stage corre- 
sponding to her size. 

We have then, in the solar system, examples of all the 
five stages of a planet's history — the first or glowing va- 
porous stage, the second or fiery stage, the stage of mid-life, 
the stage of decrepitude or old age, and finally the stage of 
death, when no life can any longer be borne on the planet's 
dry, desolate, and dead surface. 



8o MYSTERIES OF TIME AND SPACE. 



THE SUN AS A PERPETUAL MACHINE. 

Among the problems which have proved most perplexing to 
astronomers and physicists, there are few which surpass in 
difficulty the problem of the conservation of solar energy. 
The mighty orb of the sun pours forth in each second of 
time as much heat as would come from the burning of 
16,436 millions of millions of tons of the best anthracite 
coal. Yet of all this tremendous radiation of heat all the 
planets together receive less than one 230,000,000th part. 
When we consider this it seems at first view as though there 
were some degree of truth in the saying that in the universe 
' we find Nature upsetting a gallon to fill a wine-glass. ; 

In company with this great mystery of seeming waste 
comes the yet more difficult problem, How to explain the 
apparent continuance of solar light and heat during millions 
of years. We know from the results of geological research 
that the earth has been exposed to the action of the solar 
rays with their present activity during at least a hundred 
million years. Yet it is difficult to see how on any hypo- 
thesis of the generation of solar heat, or by combining 
together all possible modes of heat-generation, a supply for 
more than 20 millions of years in the past and a possible 
supply for as long a period in the future can be accounted 
for. 

It is well known, of course, to all who are likely to read 
these lines that Dr. Siemens is the inventor of what is called 
the regenerative furnace, in which the heat, which in ordi- 
nary furnaces goes up the furnace chimney and is wasted, is 



THE SUN AS A PERPETUAL MACHINE. 81 

carried back and made to do work. His theory of the 
solar heat seems to have been suggested by this invention 
of his own. The enormous waste of solar energy which 
unquestionably takes place if those rays which do not fall on 
planets do not do their proper work is obviated, he believes, 
by a contrivance (if one may so speak) which enables them 
to store up work in interstellar space, which is presently 
brought back to its source for fresh use. According to this 
view, and it is this which renders the theory attractive to 
many who had been appalled by the seemingly wanton 
waste of all save the minutest fraction of the sun's heat, 
only those rays which fall on the planets are actually and 
finally used up, so that, if the theory be true, the supply o " 
solar heat will last 230 millions of times longer than it 
otherwise would. Moreover, the theory has its retrospective 
side. The difficulty about the past would be removed as 
completely as what had seemed a danger in the future. If 
the theory is correct we may multiply every year during 
which it had been calculated that the supply has continued 
by 230 millions, to obtain a rough approximation to the 
time during which the sun has actually been at work at his 
present rate of emission. 

In the first place we are to assume that the gaseous 
atmospheres surrounding the sun and the planets are not 
limited, as Wollaston and others have supposed, but extend 
to indefinite distances, though of course in a very attenuated 
condition, ' Following out the molecular theory of gases 
as laid down by Clerk Maxwell, Clausius, and Thomson,' 
says Dr. Siemens, ' it would be difficult to assign a limit to 
a gaseous atmosphere in space ; and further, some writers, 
among whom I will here mention only Grove, Humboldt, 
Zollner, and Mattieu Williams, have boldly asserted the 
existence of a space filled with matter, and Newton himself, 
as Dr. Sterry Hunt tells us, has expressed views in favour of 
such an assumption.' He proceeds to notice the evidence 
in favour of this view derived from the condition in which 
meteorolites reach the earth. They are known, he says, to 

G 



82 MYSTERIES OF TIME AND SPACE. 

contain as much as six times their own volume of gases 
(taken at atmospheric pressure). In one of these meteoro- 
lites recently examined by Dr. Flight, the following per- 
centages of various gases were noted. Of carbonic oxide 
31-88, of carbonic acid gas 0*12, of hydrogen 4579, of 
olefiant gas 4*55, and of nitrogen 17*66. Here, however, I 
may note in passing that although it is quite certain these 
gases were not taken up by the meteorolite during its flight 
through our air, it by no means follows, and is indeed exceed- 
ingly improbable, if not impossible, that they were taken up 
while the meteorolite was travelling freely through inter- 
planetary or interstellar space. The general belief is that, 
as the late Professor Graham aptly expressed it, these 
bodies bring to us the hydrogen of the fixed stars (including 
our own sun) — that, in fact, they were expelled from bodies 
in a state resembling our sun, and that during their abode 
within the intensely hot orb of their parent sun, the 
hydrogen and other gases which we know to exist in the sun 
and his fellow stars were forced into (or became occluded 
in) the substance of the mass which was afterwards to 
become a meteorolite, and after long and devious wander- 
ings to reach our earth. Thus, and thus only it is believed 
by chemists, can the enormous quantity of occluded 
hydrogen in the substance of meteors be explained ; for 
nowhere else, but in the interior of suns, is there either the 
necessary heat or the necessary pressure. The absence 
of any trace of aqueous vapour, which Dr. Siemens finds 
surprising, as indeed it is on his theory, is thus readily 
accounted for ; indeed, no one would expect to find 
aqueous vapour in the substance of a meteoric mass which 
had ever had its abode in the interior of a sun. 

Dr. Siemens considers the objection that if inter- 
planetary space were occupied by gases, the planets would 
be seriously retarded. He believes that, assuming the matter 
occupying space to be an almost perfect fluid not limited by 
border surfaces, the retardation by friction through such an 
attenuated medium would be very slight indeed, even on 



THE SUN AS A PERPETUAL MACHINE. 83 

bodies moving with planetary velocities. But in this he is 
altogether mistaken. 

He notes also another objection, namely, that if the 
theory of gaseous interplanetary matter were true, the sun 
should draw to himself the greater part of the heavier gases, 
such as carbonic acid gas (carbonic anhydride), carbonic 
oxide, oxygen and nitrogen ; whereas spectroscopic analysis 
indicates at least the much greater prevalence of hydrogen, 
if not the absolute absence of these gases. Oxygen, indeed, 
has been shown by Dr. Draper to be present in the sun. 
Dr. Siemens points out that at the tremendous heat of the 
sun's mass such compound gases as carbonic oxide and 
carbonic acid could not exist as such. But he says that 
there must be regions, outside the intensely heated regions, 
where the existence of these gases would not be jeopardised 
by heat ; and in these regions accumulation of these com- 
paratively heavy gases would take place ' were it not for a 
certain counterbalancing action.' 

And here we approach what Dr. Siemens describes as a 
point of principal importance in his argument, upon the 
proof of which his further conclusions must depend. 

The sun rotates on his axis, completing one revolution 
in about twenty-five days, and f the sun's diameter being 
taken at 882,000 miles' (it is really considerably less than 
this, however), ' it follows that the tangential velocity 
amounts to 1*25 miles per second, or to 4-41 times the 
tangential velocity of our earth. This high rotative velocity 
of the sun must cause' (it is Dr. Siemens who speaks) * an 
equatorial rise of the solar atmosphere to which Mairan, in 
1 73 1, attributed the appearance of the zodiacal light/ He 
goes on to consider Laplace's objection to this explanation 
on the ground that the zodiacal light extends to a distance 
from the sun exceeding our own distance, whereas the 
equatorial rise of the solar atmosphere due to its rotation 
could not exceed 9-2oths of the distance of Mercury. But 
Dr. Siemens finds in the existence of a medium of un- 
bounded extension an answer to Laplace's objection. ' In 



84 MYSTERIES OF TIME AND SPACE. 

this case/ he says, ' pressures would be balanced all round, 
and the sun would act mechanically upon the floating 
matter surrounding it, in the manner of a fan, drawing it 
towards itself upon the solar surfaces, and projecting it out- 
wards in a continuous disc-like stream.' 

Now it is just at this critical part of the theory, on the 
proof of which the further conclusions of the theorist must 
depend, that dynamical considerations throw doubt, and 
something more than doubt, upon the entire speculation. 

We have a supposed fan-like action, by which hydrogen, 
hydro-carbons, and oxygen are supposed to be drawn in 
enormous quantities towards the polar surface of the sun. 
During their approach they are supposed to pass from their 
condition of extreme attenuation and extreme cold, to that 
of compression, accompanied with rise of temperature, un- 
til on approaching the photosphere they burst into flame, 
giving rise to a great development of heat, and a tempera- 
ture commensurate with their point of dissociation at the 
solar density. The result of their combustion is aqueous 
vapour and carbonic acid or carbonic oxide, according to 
the sufficiency or insufficiency of oxygen present to com- 
plete the combustion, and these products of combustion 
in yielding to the influence of centrifugal force will flow 
towards the solar equator. . . . So much we may regard as 
possible, though much would have to be proved before it 
could be regarded as probable. But Dr. Siemens goes on 
to say that the matter thus carried towards the solar equator 
will be thence projected into space. 

Now there can be nothing simpler than the con- 
siderations on which such projection into space would 
depend. The question whether a body moving in a par- 
ticular way at any part of the sun's surface will travel 
outwards into space, or will not travel outwards, can be 
answered according to certain very definite laws. If the 
velocity of its motion exceeds a certain amount, the body 
will recede from the sun ; if it falls short of that amount 
the body will tend to approach the sun's centre j if the 



THE SUN AS A PERPETUAL MACHINE. 85 

body has just that velocity, then the body will neither 
recede nor approach. Now it suggests the idea of tre- 
mendous centrifugal tendency to say that at the sun's 
equator the velocity is 4*41 times the tangential velocity 
(at the equator) of our earth. Bodies do not fly from our 
earth's equator on account of the enormous tangential 
velocity there (more than a thousand miles per hour) ; but 
it is easy to imagine, as Dr. Siemens evidently does, that 
with the much greater velocity at the sun's equator there 
may be such a tendency as his theory requires. What is, 
however, the actual state of the case ? Centrifugal tendency 
varies in the first place as the square of the velocity ; and 
squaring 4*41 we get 19*45 ' so that if our earth were to 
rotate 4*41 times as fast as she actually does, the centrifugal 
force at the equator would be increased 19*45 times. Even 
that would not be nearly enough to make bodies fly off at 
the equator. (In fact it can easily be shown that for bodies 
just to becQme weightless at the equator the earth should 
rotate in ij hours, or sixteen times as fast as at present.) 
But this is only a small part of the matter. Centrifugal 
force not only varies as the square of the velocity, but 
inversely as the distance from the centre of motion. So 
that as the sun's diameter exceeds the earth's about 108 
times, centrifugal tendency at his equator is diminished in 
this degree so far as this particular circumstance is con- 
cerned. Increasing the tendency 19*45 times and reducing 
it 108 times, means in all reducing it to about two-elevenths 
of the centrifugal tendency at the earth's equator. Yet even 
this is not all. Not only is the centrifugal tendency at the 
sun's equator less than a fifth that at the earth's equator, 
which diminishes by a very small part the force of terrestrial 
gravity, but the centripetal tendency due to the sun's attrac- 
tive force is very much greater at the sun's surface than 
terrestrial gravity at the earth's equator. It is roughly about 
twenty-seven times as great. Thus the centripetal tendency 
of matter at the sun's equator is very much greater (many 
hundreds of times greater) than its centrifugal tendency ; 



S6 MYSTERIES OF TIME AND . SPACE. 

and there is not the slightest possibility of matter being 
projected into space from the sun's surface by centrifugal 
tendency. Nor is there any part of the sun's mass where 
the centrifugal tendency is greater than at the surface near 
the equator. So that whatever else the sun may be doing 
to utilise his mighty energies, he is certainly not throwing 
off matter constantly from his equatorial regions, as Dr. 
Siemens' theory requires. 

This being so, the theory failing thus in a matter abso- 
lutely essential to its validity, we may feel less tempted than 
perhaps we otherwise might be, to endeavour to overlook 
other difficulties, though these on careful consideration 
appear scarcely less decisive. It might perhaps appear a 
work of supererogation to consider difficulties when we have 
already noted an impossibility. But some perhaps will con- 
sider that although the sun may not, after drawing to him- 
self the matter occupying space, reject it from him in the 
manner supposed, he may reject it in some other manner. 
If so there might still be reason for inquiring how far it is 
likely that the sun's rays may be utilised when falling on the 
matter occupying space, in the way suggested by Dr. Siemens. 

Let us then grant the existence in interplanetary space 
of those products of combustion which Dr. Siemens sup- 
poses to be constantly projected from the sun, and let us 
inquire with him what would become of them. At a first 
view it seems as though they must gradually change the 
condition of the matter which had formed part of stars and 
suns, by rendering that matter neutral. But Dr. Siemens 
endeavours to show the possibility, nay, the probability, 
that solar radiation would under these circumstances step in 
to bring back the combined materials to a condition of 
separation by a process of dissociation, carried into effect at 
the expense of that solar energy which is now supposed to 
be lost to our planetary system. 

Dr. Siemens points out that the temperature at which 
the dissociation of different compounds is effected depends 
on the pressure. Thus at a temperature of 2,800° Centi- 



THE SUN AS A PERPETUAL MACHINE. S7 

grade only one-half of the vapour of water at atmospheric 
pressure remains as aqueous vapour, the remaining half 
being found as a mechanical mixture of hydrogen and 
oxygen. But with the pressure the temperature of disso- 
ciation rises and falls, It is therefore conceivable, he says, 
that the temperature of the solar photosphere may be raised 
by combustion to a temperature exceeding 2,800° Centi- 
grade, whereas in interstellar and interplanetary space dis- 
sociation may be effected at a much lower temperature. 
Some experiments by Dr. Siemens appear to show that at 
the small pressure which we may conceive to exist in space, 
the sun's radiation may suffice to produce dissociation either 
of aqueous vapour or of carbonic acid gas. Employing 
glass tubes furnished with platinum electrodes, and filled 
with aqueous vapour, he reduced the pressure to i-g^th of 
an atmosphere, the temperature being reduced to 32° Centi- 
grade. When so cooled, no electric discharge took place 
on connecting the two electrodes with a small induction 
coil. He then exposed the end of the tube projecting out 
of the freezing mixture, backed by white paper, to solar 
radiation on a clear summer's day for several hours, when 
upon again connecting up to the inductorium, a discharge, 
apparently that of a hydrogen vacuum, was obtained. 
' This experiment being repeated, furnished,' says Dr. 
Siemens, 'unmistakable evidence I thought that aqueous 
vapour had been dissociated by exposure to solar radiation.' 
When carbonic acid gas was similarly treated, less trust- 
worthy results were obtained. 'Not satisfied with these 
qualitative results, I made arrangements to collect the per- 
manent gases so produced, by means of a Sprengel pump, 
but was prevented by lack of time from pursuing the 
inquiry, which I purpose, however,' adds Dr. Siemens, ' to 
resume shortly, being of opinion that, independently of 
my present speculation, the experiments may prove useful 
in extending our knowledge regarding the laws of disso- 
ciation.' 

The idea is, then, that solar radiation acting on the 



88 MYSTERIES OF TIME AND SPACE. 

aqueous vapour and carbonic acid gas, and other compound 
gases supposed to occupy interplanetary and interstellar 
space, may dissociate such compounds, and that solar 
energy may thus be utilised, instead of being wasted in the 
enormous degree in which it appears to be, according to 
what has been shown above. 

Now it appears to me somewhat bold to assume that 
what happens in the case of aqueous vapour or carbonic 
acid enclosed in a tube and exposed to solar radiation, 
would happen to such vapour exposed to the same radiation 
in free space. But there is a more serious objection, I take 
it, than this, to Dr. Siemens' ingenious system for the utili- 
sation of solar energy. If the rays of heat (and light) are 
thus utilised within the solar domain, regarding that if we 
please as extending many times further than the orbit of 
Neptune, they have either done their work and have been 
completely utilised, or they have not. If they have done 
their work, these rays proceed no further, and the sun would 
therefore be invisible from any point outside his own 
domain. (For we must not fall into the mistake of sup- 
posing that light and heat can be considered separately in 
this inquiry : those solar rays which give us what we call 
light, give us also a large quantity of the solar heat, and the 
mystery of seemingly infinite waste would remain, even if 
we supposed that only those heat rays which are not also 
light rays were utilised in the way supposed. Apart from 
this, Dr. Siemens specially shows how the light rays act in 
accordance with his views.) Now what is true of our sun is 
true of the other suns, the stars. They also ought to be 
invisible outside their several domains. But as a matter of 
fact they are visible. If, on the other hand, the solar rays 
have not done their work in traversing what may be regarded 
as the solar domain, the mystery of infinite waste is not 
removed, scarcely even diminished, by Dr. Siemens' theory. 
If those other suns, the stars, are able to send across the 
vast distances which separate us from them, such supplies 
of light (to say nothing of stellar heat, which Huggins and 



THE SUN AS A PERPETUAL MACHINE. 89 

others have measured) that by measuring it we can say that 
all of them are suns like our own, but many far larger and 
giving out much more light than he, — what is the amount 
of work which we can suppose the stellar rays to have done 
on their way ? If they have done much (in proportion to 
the total quantity which they are capable of doing), then 
the stars must be very much larger, brighter, and hotter 
than we suppose them to be, and already we regard them 
as the rivals, and something more than the rivals, of our 
sun. If they have done little, the mystery of infinite waste 
remains. 

But indeed, apart from the considerations last urged, it 
is certain that even if the whole of interstellar space were 
filled with matter dissociated by solar rays (that is by the 
rays which all suns are continually pouring forth), even 
then those rays would have been to all intents and purposes 
wasted \ for suns never could gather in more than the 
minutest fraction of the matter thus permeating space. We 
cannot adopt Dr. Siemens' theory, supposing it otherwise 
tenable, as a means of utilising solar and stellar energy, 
unless we supposed the work done by the light and heat of 
suns to be done close to those orbs, certainly far within the 
orbits of their outer planets, for otherwise the matter pre- 
pared for fuel by the action of the rays could never be 
gathered in, or the products of combustion expelled, within 
reasonable time, throughout the domain thus affected. But 
we know certainly that within such relatively insignificant 
domains the stellar rays are not used up, for we see the 
stars shining, though we lie millions of times farther away 
than any conceivable limits of such domains. We know it 
in the case of our own sun, because we see the planets 
Saturn, Mars, and Neptune shining with light which has 
reached them from the sun. In the case of the Siemens' 
regenerative furnace, we know that the heat is utilised in the 
particular manner intended, not only because we find the 
heat so saved doing its proper work, but because we find 
that this heat no longer goes idly up the furnace chimney 



90 MYSTERIES OF TIME AND SPACE. 

as before. The heat cannot be doing its full work in the 
furnace if part goes up the furnace chimney ; but also, part 
cannot be going up the furnace chimney if the heat is 
doing its full work. This, however, is what Dr. Siemens' 
theory requires the solar heat to do. It is to be continually 
utilised in dissociating compound vapours in interplanetary 
space, although it is continually passing beyond interplanet- 
ary space to shine through interstellar space, and to show 
our sun as a star to worlds circling round his fellow-stars the 
suns. We have in fact the fallacy of the perpetual motion 
in a modified form. 

Parts of Dr. Siemens' reasoning remain tenable, how- 
ever, even when the centrifugal projective force (which has 
no existence) is removed, and when the perpetual utilisation 
of stellar rays is shown to be inconsistent with their per- 
petual passage with undiminished brightness through inter- 
stellar space. 

Dr. Siemens' reasoning respecting the zodiacal light, for 
instance, is sound, though the theory with which it is asso- 
ciated is not so. Astronomers do not and cannot accept 
the views of Mairan, which are simply inconsistent with the 
known laws of dynamics. But there is every reason for 
regarding the zodiacal as consisting in the main of meteoro- 
lithic masses, a sort of cosmical dust, rushing through inter- 
planetary space with planetary velocities. To such matter, 
assuming, as we well may, that space really is occupied by 
attenuated vapours, the following reasoning applies with 
scarcely the change of a word (by which, however, I do not 
mean that the opinions expressed as probably or possibly 
true are really and necessarily so). The luminosity of the 
zodiacal ' would be attributable to particles of dust, emitting 
light reflected from the sun, or by phosphorescence ' (this 
last may be seriously questioned). ' But there is another 
cause for luminosity of these particles, which may deserve a 
passing consideration. Each particle would be electrified 
by gaseous friction in its acceleration, and its electric ten- 
sion would be vastly increased in its forcible removal, in 



THE SUN AS A PERPETUAL MACHINE. 91 

the same way as the fine dust of the desert has been 
observed by Werner Siemens to be in a state of high electri- 
fication on the apex of the Cheops Pyramid. Would not 
the zodiacal light also find explanation by slow electric 
discharges backward from the dust towards the sun ? ' 

Take, again, the phenomena of comets which still 
remain among the greatest of nature's mysteries. We have 
reason to believe — though Dr. Siemens goes a little beyond 
the truth in saying astronomical physicists assert — that the 
nucleus of a comet consists of an aggregation of stones 
similar to meteorolites. Adopting this view, and assuming 
that these stones have absorbed somewhere (not necessarily 
1 in stellar space,' as Dr. Siemens suggests) gases to the 
amount of six times their volume (taken at atmospheric 
pressure), we may ask with Dr. Siemens, what will be the 
effect of such a mass of stone advancing towards the sun at 
a velocity reaching in perihelion the prodigious rate of 366 
miles per second (as observed in the comet of 1843), being 
twenty-three times our orbital rate of motion ? ' It appears 
evident that the entry of such a divided mass into a com- 
paratively dense atmosphere must be accompanied by a rise 
of temperature by frictional resistance, aided by attractive 
condensation. At a certain point the increase of tempera- 
ture must cause ignition, and the heat thus produced must 
drive out the occluded gases, which in an atmosphere 3,000 
times less dense than that of our earth would produce 
(6x3,000=) 18,000 times the volume of the stones them- 
selves. These gases would issue forth in all directions, 
but would remain unobserved except in that of motion, in 
which they would meet the interplanetary atmosphere with 
the compound velocity and from a zone of intense com- 
bustion, such as Dr. Huggins has lately observed to sur- 
round one side of the nucleus, evidently the side of forward 
motion. The nucleus would thus emit original light, 
whereas the tail may be supposed to consist of stellar dust 
rendered luminous by reflex action produced by the light 
of the sun and comet combined.' (This assumption respect- 



92 MYSTERIES OF TIME AND SPACE. 

ing the tail is, however, untenable, being based on a mis- 
apprehension of the distinction between a comet's tail and 
its train of meteoric attendants.) 

These views respecting the zodiacal light and comets 
are independent in the main of those parts of Dr. Siemens' 
views which are manifestly inadmissible. They seem to 
accord well with possibilities if not with probabilities. 

A similar remark applies to two of the fundamental 
conditions of Dr. Siemens' ingenious theory. We may 
admit the possibility that the aqueous vapour and carbon 
compounds are present in stellar or interplanetary space ; 
we may concede, though perhaps not quite so readily, that 
these gaseous compounds are capable of being dissociated 
by radiant solar energy while in a state of extreme attenua- 
tion. What we cannot admit, simply because it is incon- 
sistent with known laws, is the third condition, • That these 
dissociated vapours are capable of being compressed into 
the solar photosphere by a process of interchange with an 
equal amount of reassociated vapours, this interchange being 
effected by the centrifugal action of the sun itself.' As this 
condition is essential to the theory itself, we are compelled, 
regretfully perhaps, but still unhesitatingly, to give up that 
satisfaction which, as Dr. Siemens remarks, we should gain, 
could we believe that our solar system need ' no longer 
impress us with the idea of prodigious waste through the 
dissipation of energy into space, but rather with that of 
well-ordered, self-sustaining action, capable of perpetuating 
solar radiation to the remotest future/ Yet though not in 
this way, to this end all thoughtful study of the mechanism 
of the universe seems unquestionably to tend ; not by cen- 
trifugal tendencies of the kind imagined, for none such 
exist • not by work which, viewed in reference to the uni- 
verse as we know it, means endless production without 
exhaustion ; but in other ways (associating perhaps our 
visible universe with others, permeating it as the ether of 
space permeates the densest solids, and in turn with others 



THE SUN AS A PERPETUAL MACHINE. 93 

so permeated by it) there may be that constant interchange, 
that perpetual harmony, of which Goethe sung — 

See all things with each other blending, 

Each to all its being lending, 

Each on all in turn depending : 

Heavenly ministers descending, 

And again to Heaven uptending, 

Floating, mingling, interweaving, 

Rising, sinking, and receiving — 

Each from each, while each is giving 

On to each, and each relieving 

Each — the pails of gold. The living 

Current through the air is heaving ; 

Breathing blessings see them bending, 

Balanced worlds from change defending, 

While everywhere diffused is harmony unending. 



94 MYSTERIES OF TIME AND SPACE. 



THE SUN'S CORONA. 

One after another the mysterious problems presented by the 
sun to man's contemplation have been solved by astrono- 
mers. We have learned what are the substances which 
compose his giant bulk. We know much respecting the 
condition in which those substances exist. The strange red 
prominences which are seen round the black disc of the 
moon in total eclipse, ' like garnets round a brooch of jet,' 
have not only been interpreted, but our astronomers, calling 
in to their aid the subtle powers of the most wonderful 
instrument of research yet devised by man, have been 
enabled to discern these objects when the sun is shining 
with full splendour in the heavens — nay, even to measure 
their motion, and to gauge the pressure exerted by the gases 
which compose their substance. But one great problem yet 
remains unsolved. When the sun's orb is hidden in total 
eclipse, there bursts suddenly into view a crown or glory of 
light, resembling the nimbus which painters place around the 
heads of saints. Sometimes presenting the appearance of 
a uniform circular halo, at others radiated and even irregu- 
lar in aspect, this striking phenomenon had long attracted 
the attention and invited the curiosity of astronomers. But 
recently, owing to the nature of the information obtained 
respecting the sun's substance and the coloured flames 
which play over his surface, the corona has been regarded 
with much greater interest. 

The corona was known to astronomers long before those 
coloured prominences which have recently received so much 



THE SUN'S CORONA. 95 

attention. It has even been supposed that Philostratus 
refers to the appearance of this object where he remarks, in 
his ' Life of Apollonius,' that l there appeared in the 
heavens ' — shortly before the death of Domitian — ' a prodigy 
of the following nature — a certain corona, resembling the 
iris, surrounded the orb of the sun and obscured his light.' 
One might conceive that there was no reference here to a 
total eclipse of the sun ; but Philostratus remarks farther 
on, that the darkness was like that of night, a circumstance 
which leaves little doubt that a solar eclipse had taken 
place. 

It is, in fact, worthy of remark, that the light of the 
corona often misled the observers of total eclipses to suppose 
that, in reality, a portion of the sun had remained uncovered. 
Kepler was at the pains to write a treatise to prove that 
certain eclipses, supposed to be only annular, had, in 
reality, been total. A year after he had published this trea- 
tise, he himself had an opportunity of witnessing the total 
eclipse visible at Naples in 1605, respecting which he remarks, 
that ' the whole body of the sun was completely covered for 
a short time, but around it there shone a brilliant light of a 
reddish hue and uniform breadth, which occupied a consider- 
able part of the heavens.' 

From this time scarcely a singe total eclipse has occurred 
during which the aspect and dimensions of the corona have 
not been noted. It would be easy to fill a volume with the 
various observations which have thus been recorded. For 
our purpose, it will be convenient to select those accounts 
which indicate the most important peculiarities of the corona, 
and especially those which may help us to ascertain the real 
nature of the object. 

One of the earliest accounts of this nature is that given 
by Dr. Wyberd of the total eclipse of March 29, 1652. 
' When the sun was reduced to a narrow crescent of light,' 
he remarks, ' the moon all at once threw herself within the 
margin of the solar disc'— (a peculiarity which has been 
observed under favourable circumstances by others, and is, 



96 MYSTERIES OF TIME AND SPACE. 

of course, only apparent) — 'with such agility, that she 
seemed to revolve like an upper millstone, affording a plea- 
sant spectacle of rotary motion. In reality, however, the 
sun was totally eclipsed, and the appearance was due to a 
corona of light round the moon, arising from some unknown 
cause. It had a uniform breadth of half a digit or a third of 
a digit at least ; it emitted a bright and radiating light, and 
appeared concentric with the sun and moon ' when the cen- 
tres of the two discs were at their nearest. 

It will presently be seen that the extent of the corona on 
this occasion was far less than during many modern eclipses ; 
in fact, Dr. Wyberd's account would seem to indicate that 
he only noticed the brighter part of the corona which lies 
close by the black disc of the moon. Otherwise the ex- 
tent of the corona on this occasion was exceptionally small. 
Strangely enough, the next account we have to refer to 
assigns to the corona an exceptionally large extension from 
the sun. 

During the eclipse of May 12, 1706, MM. Plantade and 
Capies saw a very bright ring of white light surrounding the 
eclipsed sun, and extending to a distance equal to about a 
tenth of the moon's apparent diameter. This was, in all 
probability, that brighter portion of the corona which Dr. 
Wyberd saw. Outside this brilliant ring of light a fainter 
light was seen, which faded off insensibly until — at a dis- 
tance from the sun equal to about eight times his apparent 
diameter — the light was lost in the obscure background of 
the sky. 

This observation serves very well to indicate the 
interest and importance attaching to the solution of the 
problem presented by the corona. We shall see presently 
that a question exists whether the corona is, on the one 
hand, a solar appendage, or, on the other, a phenomenon 
due merely to the passage of the sun's rays through our own 
atmosphere. The observation just described would, in the 
one case, indicate that the object has a real extension enor- 
mously exceeding that of any known celestial object — save 



THE SUN'S CORONA. 97 

perhaps the tails of certain comets — while in the other case, 
the corona would have no more scientific importance than 
those long radial beams formed by the light of the sun shin- 
ing through a bank of clouds. Enormous as is the bulk of 
the sun — so enormous that the earth on which we live sinks 
into utter nothingness by comparison — the actual extent of 
space filled by the coronal light, on the former supposition, 
could exceed the volume of the sun more than two thousand 
times ! 

It is not without some little shame that astronomers 
refer to the great total eclipse of 17 15. Although this 
eclipse was visible in England, and though it occurred in 
the time of so great an astronomer as Halley, no adequate 
preparations were made for observing it. Coates, indeed — ■ 
a practical astronomer, whose observations would have had 
a high value — was 'oppressed with too much company,' 
Halley tells us, to pay special attention to the eclipse. Halley 
himself made a few commonplace notes on the phenomena 
presented by the totally eclipsed sun, but we learn nothing 
new from them respecting the corona. 

Nor were the French astronomers more energetic in 1724. 
But one observation made by Maraldi is worth noticing. 
He perceived that at the beginning of the eclipse the corona 
was clearly broader on the side towards which the moon 
was advancing than on the opposite side, while at the end 
of the eclipse the reverse was the case. This would seem 
to show that the corona is a solar appendage, since the moon 
thus seemed to traverse the corona precisely as she traversed 
the sun. 

The observation made by Maraldi was confirmed by 
several who observed the total eclipse of 1733 in Sweden. 
A special interest attaches to this eclipse, because instead 
of being observed only by astronomers, it was watched by 
a large number of persons invited to the work by the Royal 
Society of Sweden. As many of those who propose to join 
the expedition to view the eclipse of next December have 
decided to direct their attention to the general aspect of the 

u 



98 MYSTERIES OF TIME AND SPACE. 

corona, it is interesting to inquire how far such observations 
are likely to add to our knowledge. In this respect the 
Swedish narrative is most encouraging. At Catherinesholm, 
the pastor of Forshem noticed that the ring of light which 
appeared round the black disc of the moon was of a reddish 
colour, an observation confirmed by Vallerius, another pas- 
tor, who noticed, however, that a considerable distance from 
the sun the ring appeared of a greenish hue. The pastor of 
Smoland states that ' during the total obscuration the edge 
of the moon's disc resembled gilded brass, and that the faint 
ring around it emitted rays in an upward as well as in a 
downward direction, similar to those seen beneath the sun 
when a shower of rain is impending.' The mathematical 
lecturer in the Academy of Charlestadt, M. Edstrom, ob- 
served these rays with special attention, and remarks respect- 
ing them that ' they plainly maintained the same position 
until they vanished along with the ring upon the reappear- 
ance of the sun.' On the other hand, the ring as seen at 
Lincopia seemed to have no rays. 

It is important to inquire whether this difference in the 
aspect of the corona, as seen at different stations, is due to 
the condition of the air, the eyesight of the observer, or other 
such causes. For clearly, if the observer at Lincopia saw an 
object really different from that seen by Edstrom, it would 
follow that the corona is a phenomenon of our own atmo- 
sphere and not a solar appendage. On other occasions a 
like difference has been recorded in the aspect of the corona 
as seen at different stations ; but we do not remember any 
observations which seem calculated to resolve the question 
just suggested, until the great total eclipse observed last year 
in America. It is easy to see that, whatever theory of the 
corona we adopt, the condition of the atmosphere might be 
expected to affect the aspect of the ring. For obviously this 
would happen if the coronal light is merely due to the 
illumination of our atmosphere ; while, if the light comes 
from beyond our atmosphere, it would still be brighter or 
fainter according as the air was more or less clear, The only 



THE SUN'S CORONA. 99 

convincing form of evidence would be such as showed that 
some peculiarity of figure, noticed when the ring was seen 
under unfavourable atmospheric conditions, remained recog- 
nisable notwithstanding a great increase in the apparent 
extent of the ring, when seen at some distant station, under 
more favourable circumstances. 

Now during the great eclipse of the year 1869, very re- 
markable evidence was given, fulfilling these very conditions. 

In the first place, all the astronomers who observed the 
eclipse along the whole path of the shadow, from whence it 
first fell upon America far in the North-west to the point 
where it left the American continent and fell upon the Atlan- 
tic, noticed the singularly quadrilateral aspect of the corona. 
This was not only observed with the naked eye, but by tele- 
scopists ; and in one instance photography recorded the 
peculiarity most satisfactorily. But this four-cornered aspect 
belonged only to a portion of the coronal light lying rela- 
tively close to the sun. The most distant corner of the four 
lay at a distance from the moon's disc scarcely exceeding 
half the moon's apparent diameter. Outside the cornered 
figure lay a faint glare of light, which seemed to most ob- 
servers to merge uniformly and gradually into the dark tints 
of the sky far away from the eclipsed sun. 

But there was one party of observers who were stationed 
above those lower and denser regions of the atmosphere 
which are most effective in obstructing the passage of light, 
and especially of light so faint as that which comes from the 
outer parts of the corona. General Myer, Colonel Winthrop, 
and others ascended to the summit of White Top Mountain, 
near Abingdon in Virginia, and thence, at a height of some 
5,500 feet above the level of the sea, and immersed so much 
more deeply in the shadow of the moon than the observers 
at low levels, they had an opportunity of witnessing the im- 
posing phenomena presented during a total eclipse of the 
sun. The account they give of the corona becomes, under 
these circumstances, most instructive. 'To the unaided 
eye,' says General Myer, ' the eclipse presented, during the 

H 2 



ioo MYSTERIES OF TIME AND SPACE. 

total obscuration, a vision magnificent beyond description. 
As a centre stood the full and intensely black disc of the 
moon, surrounded by an aureola of soft bright light, through 
which shot out, as if from the circumference of the moon, 
straight massive silvery rays, seeming distinct and separate 
from each other, to a distance of two or three diameters of the 
lunar disc ; the whole spectacle showing as upon a back- 
ground of diffused rose-coloured light. . .'\ The silvery rays 
were longest and most prominent at four points of the circum- 
ference — two upon the upper, and two upon the lower por- 
tion, apparently equidistant from each other ... giving 
the spectacle a quadrilateral form. The angles of the quad- 
rangle were about opposite the north-eastern, north-western, 
south-eastern, and south-western points of the disc' (an 
arrangement corresponding precisely with the observations 
made at lower levels). ' There was no motion of the rays 
— they seemed concentric' 

Nothing, as it should seem, could be more convincing 
than the evidence given by this observation. The radial 
extensions which, to the observer near the sea-level, reached 
only to a distance from the moon's edge equalling about 
half the moon's diameter, were recognised at the higher 
station as rays four times as long. The influence of the 
atmosphere in blotting out, so to speak, the fainter portions 
of the corona is thus made manifest, and so far the evi- 
dence strongly favours (to say the least) the supposition that 
the corona is something lying much farther from us than the 
limits of the earth's atmosphere. 

Let us return, however, to the records of earlier eclipses. 
Strangely enough the next we have to deal with corre- 
sponds very closely with the American eclipse as respects 
the appearance presented by the corona. * The most re- 
markable feature exhibited by the corona,' remarks Professor 
Grant, speaking of the eclipse of February 1766, 'consisted 
of four luminous expansions, separated from each other by 
equal intervals.' 

The. Spanish admiral, Don Antonio d' Alloa, gives an 



THE SUN'S CORONA. 101 

interesting account of the appearance presented by the 
corona during the total eclipse of 1778. He states that 
' five or six seconds after the commencement of the total 
obscuration, a brilliant luminous circle was seen surrounding 
the moon, which became more vivid as the centre of that 
body continued to approach the centre of the sun. About 
the middle of the eclipse, its breadth was equal to one-sixth 
of the moon's diameter. There appeared issuing from it a 
great number of rays of unequal length, which could be dis- 
cerned to a distance equal to the lunar diameter. It seemed 
to be endued with a rapid rotatory motion, which caused it 
to resemble a firework turning round its centre. The colour 
of the light was not uniform throughout the whole breadth 
of the ring. Towards the margin of the lunar disc it ap- 
peared of a reddish hue ; then it changed to a pale yellow, 
and from the middle to the outer border the yellow gradually 
became fainter until at length it seemed almost quite white.' 

Passing over several intermediate eclipses, we come to 
the great eclipse of 1842, remarkable on account of the 
number of eminent astronomers of all nations who took part 
in observing it. 

The most noteworthy feature in the records of this 
eclipse is the very wide range of difference in the estimates 
of the extent attained by the coronal ring. M. Petit, at 
Montpellier, estimated the width of the corona at barely 
one-fourth of the moon's diameter. Francis Baily — it was 
during this eclipse, by the way, that the phenomenon known 
as ' Baily's Beads ' was first observed with attention — con- 
sidered that the corona was about twice as wide. To Otto 
Struve, the eminent Prussian observer, the corona seemed 
yet wider, falling little short of the moon's apparent diameter 
in extension. 

It is interesting to notice these discrepancies between 
the observations of modern astronomers of repute for accu- 
racy and observing skill. It shows that the differences re- 
corded in the aspect of the corona are not due to such errors 
as unpractised observers might be expected to make. We 



102 MYSTERIES OF TIME AND SPACE. 

shall presently see the importance of thus separating truthful 
from untrustworthy observations. 

Arago made a similar observation during the progress of 
this eclipse. He remarked in one of the brighter portions 
of the corona, 'a luminous spot composed of jets entwined 
in each other, and resembling in appearance a hank of 
threads in disorder.' It is difficult to understand what this 
may have been. It would almost seem to give evidence in 
favour of a view recently put forward, that the light of the 
corona comes from innumerable streams of meteors in the 
neighbourhood of the sun. 

Some of the rays of the corona during this eclipse were 
estimated by the younger Struve as nearly eight times the 
moon's apparent diameter in length, the first instance, be it 
noted, in which a modern observation has confirmed the 
account given by MM. Plantade and Capies in 1706. 

In 185 1 the Astronomer Royal had a second opportunity 
of observing the solar corona. It affords interesting evidence 
of the variability in the appearance of this object according 
to the circumstances under which it is observed, that Airy 
recognised a distinct difference not merely in the extent 
but in the figure of the corona on this occasion. He says, 
'The corona was far broader than that which I saw in 1842. 
Roughly speaking, its breadth was little less than the moon's 
diameter, but its outline was very irregular. I did not notice 
any beams projecting from it which deserved notice as 
much more conspicuous than the others, but the whole was 
beamy, radiated in structure, and terminated — though very 
indefinitely — in a way which reminded me of the ornament 
frequently placed round a mariner's compass. Its colour 
was white, or resembling that of Venus. I saw no flickering 
or unsteadiness of light. It was not separated from the moon 
by any dark ring, nor had it any annular structure. It 
looked like a radiated luminous cloud behind the moon.' 

In i860 the Astronomer Royal again witnessed the phe- 
nomena which accompany a total eclipse of the sun ; and 
again, his evidence respecting the corona assigns to it a 



THE SUN'S CORONA. 103 

figure resembling, 'with some irregularities, the ornament 
round a compass card/ 

And now we are approaching, or, rather, we have already- 
reached the era when other modes of research than mere 
telescopic observation were to be applied to this perplexing 
phenomenon. In i860, Mr. De la Rue and the Padre 
Secchi succeeded in photographing the eclipsed sun ; and 
though but a small portion of the corona is discernible in 
their photographs, yet it is quite evident, on a careful com- 
parison of pictures taken at stations widely separated, that 
at least the brighter portion of the corona belongs to the sun. 
Where the coronal radiance is brightest or extends farthest 
in Mr. De la Rue's pictures, there also in F. Secchi's can be 
recognised corresponding peculiarities. 

Then, after a considerable interval, came the great eclipse 
of August 1868, when an effort was made to apply the 
powers of the spectroscope to the interpretation of the 
corona. It is a somewhat singular circumstance, by the bye, 
that the results of so important an observation as Major 
Tennant's spectroscopic study of the corona should be quite 
commonly misquoted— but so it is. We have before us, 
as we write, his own statement, in which are the words 
(italicised), ' What I saw was undoubtedly a continuous 
spectrum, and I saw no lines ; ' followed by the remark, 
'there may have been dark lines, of course, but with 
so faint a spectrum .... they might escape notice.' Yet 
in Roscoe's most valuable treatise on spectrum analysis 
there occur the words, 'Major Tennant states that the 
spectrum of the corona is the ordinary solar spectrum ; ' and 
the American astronomers who observed the eclipse of 1869 
repeat the statement, commenting with surprise on the fact 
that they could see no dark lines in the coronal spectrum. 

The distinction between what Major Tennant actually 
saw and what he is supposed to have seen is most important. 
If the corona gave a spectrum resembling the sun's, it would 
be reasonable to conclude that the light of the corona was 
simply reflected sunlight. But if the spectrum of the corona 



io 4 MYSTERIES OF TIME AND SPACE. 

shows no dark lines we can no longer suppose this. A burn- 
ing solid gives a rainbow-tinted spectrum of this sort, with- 
out dark lines ; and though it would not be proved, it would 
at least be rendered probable, were this the nature of the 
coronal spectrum, that the light of the corona comes from 
actually incandescent substances. 

It was hoped that the American astronomers would have 
obtained decisive results ; but a new source of perplexity 
was introduced by their observations. They satisfied them- 
selves that the coronal spectrum really was continuous ; for 
they observed it under conditions which removed all the 
doubts referred to by Major Tennant. But superposed upon 
the faint rainbow-tinted streak they saw bright lines. Pro- 
fessor Harkness saw one line only, but Professor Young saw 
three. 

Now, it is only necessary to know what is the interpreta- 
tion of a spectral bright line to understand the strange sig- 
nificance of this new observation. A glowing vapour gives 
a spectrum of bright lines. But surprising as the conclusion 
would be that the corona consists, either wholly or in part, 
of glowing vapour, it is when we consider the nature of the 
vapour indicated by the coronal bright lines that the most 
startling result of all is suggested. One of the bright lines 
corresponds in place with a line belonging to the spectrum 
of the glowing vapour of iron. This metal, which requires 
so intense a heat for its liquefaction, and, therefore, a yet 
more tremendous heat to vaporise it, would actually seem 
(from the evidence) to be present in the form of glowing 
vapour in the sun's corona. Here are the words of Pro- 
fessor Harkness — who is thoroughly familiar with the laws 
of spectroscopic analysis — announcing his acceptance of a 
conclusion as probable, which is so startling that we could 
not venture to leave it on record without such confirmation, 
lest haply the reader should regard it as simply arising from 
a misinterpretation of the evidence : — ' I consider the con- 
clusion highly probable, if not actually proved, that the 
corona is a very rarefied self-luminous atmosphere sur- 



THE SUN'S CORONA. 105 

rounding the sun, and, perhaps, principally composed of 
the incandescent vapour of iron.' And what renders the 
conclusion so much the more remarkable is that Professor 
Harkness has adduced evidence to show that the heat of 
the summits of the coloured prominences is such as would 
be insufficient to vaporise iron. The corona would be less 
heated, one would suppose, than the prominences which lie 
so much nearer to the sun. 

Such were, up to the year 1870, the observations which 
astronomers and physicists had made upon the corona. 
We have indicated in passing some of the theories sug- 
gested by special observations, but we have now to inquire 
what are the general results to which this series of re- 
searches, regarded as a whole, appears to tend. 

The theories which have been put forward by astrono- 
mers in explanation of the solar corona are not many in 
number, and some of them need not occupy us for any 
length of time, as modern researches have practically dis- 
posed of them. 

The theory that the corona is due to a lunar atmosphere 
is associated with the names of the eminent astronomers 
Kepler and Halley. It is probable that the latter would 
have been even more confident of its truth than he actually 
was, had it not been that the opinion of his great friend 
Newton was opposed to this theory. Such, at least, has been 
the interpretation placed upon Halley's remark that ' the con- 
trary sentiments of one whose judgment he should always 
revere ' caused him to feel doubtful as to Kepler's theory. 

We now know quite certainly that the moon has no 
atmosphere which could account for the appearance of the 
corona. It is doubtful whether the moon has any atmo- 
sphere at all ; but most assuredly if she have any it must 
be very limited in extent. When the moon passes over a 
star, the disappearance of the star is quite sudden • there is 
no sign whatever of that gradual diminution of the star's 
light which would undoubtedly be recognised if the moon 
had an atmosphere of appreciable extent. 



106 MYSTERIES OF TIME AND SPACE. 

The French astronomers, La Hire and De Lisle, put 
forward two theories, which are now obviously untenable. 
According to each theory, the appearance of the corona is 
caused by an action on the sun's rays, that action taking 
place at the edge of the moon's disc — the difference between 
the two theories being that La Hire ascribed the action to 
the inequalities of the moon's surface and their power of 
reflecting the solar rays, while De Lisle supposed that the 
sun's rays were diffracted at the moon's edge. We owe to 
Baden Powell and Sir David Brewster the disproof of De 
Lisle's theory, De Lisle himself having disposed of La 
Hire's. 

There remains, then, only one theory to consider, the 
theory, viz., that the corona is a true solar appendage, and 
one of the most remarkable features in the universe (for 
the other, that the corona is simply a terrestrial pheno- 
menon, due to the passage of the sun's rays through our 
own atmosphere, has never been supported by anyone com- 
petent to express an opinion on such matters, and has 
now disappeared even from the writings of those who 
advanced it). 

The corona cannot be a solar atmosphere. It will be 
obvious that if the corona were such an atmosphere, it 
would exert a pressure upon the sun's surface corresponding 
to that pressure which our own atmosphere exerts upon the 
surface of the earth. But then the pressure exerted by the 
coronal atmosphere would be incalculably greater. Our own 
atmosphere, we have reason to believe, does not extend 
much more than ioo miles above the sea-level. Now the 
corona is visible, under favourable circumstances, at a dis- 
tance from the sun equal to his own diameter — setting aside 
all considerations of the radial projections. In other words, 
it certainly does not extend less than 850,000 miles from his 
surface. Regarded as an atmosphere, therefore, the corona 
is certainly not less than 8,000 times as deep as our own. 
On this account alone the pressure it would exert would be 
enormously greater. For it is to be noted that the pressure 



THE SUN'S CORONA. 107 

exerted by our air would not be merely doubled were the 
height of the atmosphere doubled, trebled were that height 
trebled, and so on, but would increase at a much more 
rapid rate. If a mine were sunk into the earth in order to 
measure the increase of atmospheric pressure with depth, 
instead of a depth of 100 miles being required in order to 
have a double pressure, only 3^ miles would be needed. At 
the bottom of a mine 7 miles deep the pressure would be 
four times as great as at the sea-level ; 10 \ miles deep the 
pressure would be eight times as great ; 14 miles deep the 
pressure would be sixteen times as great, and so on, like 
the expense of the miser's grave, ' doubling as we descend ' 
for every 3^ miles. It requires no great knowledge of arith- 
metic to see that the pressure at a depth of 100 miles or so 
would be millions of times greater than that at the sea- 
level. 1 It will be seen, therefore, how inconceivably great 
the pressure exerted by a solar atmosphere some 8,000 
times as deep as ours would necessarily be, let the nature 
of the gases composing it be what it may. 

But even this is not all. We have hitherto only com- 
pared the height of the supposed solar atmosphere with that 
of the earth's. We must not forget that the sun's attractive 
energy so enormously exceeds the earth's that even though 
his atmosphere were no deeper than ours (and similarly 
constituted) the pressure exerted on his surface would be 
enormously increased. If a man could be placed on the 
solar surface his own weight would crush him as effectually 
as though while on the earth a weight of a couple of tons 
were heaped upon him. In precisely the same way the 
pressure of the solar atmosphere is increased by the enor- 
mous force with which the sun drags towards himself every 
particle composing that atmosphere. 

Now it happens that we know quite well that the pres- 
sure exerted by the real solar atmosphere, even close by the 



1 The actual number representing the proportionate pressure would 
consist of no less than nine figures, being very nearly two hundred 
millions. 



108 MYSTERIES OF TIME AND SPACE, 

bright surface which forms the visible globe of the sun, is 
nothing like so great as it would be if the corona formed 
part of that atmosphere. The bright lines constituting the 
spectrum of the coloured prominences would be many times 
thicker than they are if the pressure were so great ; for 
spectroscopists have found, by means of experiments made 
in the laboratory, that with increase of pressure the spectral 
bright lines of a gas increase in thickness. 

Here, then, we have the most conclusive proof possible 
that the corona is not a solar atmosphere. 

But, on the other hand, those who argue that the corona 
is a solar appendage, ask how it happens, if the phenomenon 
is due to the illumination of our own atmosphere, that the 
moon looks black in the very heart of this illumination. If 
our air were illuminated, its light would extend over the 
moon also — since the moon lies so far beyond its limits ; 
whereas the moon is as a dark disc on the background of 
the coronal light. This very word background, obviously 
applicable to the corona as actually seen, indicates that the 
source of the coronal light is beyond the moon. 

Here, then (to mention no other considerations), we 
have the most conclusive evidence that the corona is not 
a phenomenon of our own atmosphere. 

But then the corona is clearly somewhere and something. 
If its light comes from beyond the moon, we need not doubt 
that it comes from the sun's neighbourhood ; and again, if 
the corona is not a solar atmosphere, we can scarcely doubt 
that it is a solar appendage. It would seem to follow that 
the corona is due to bodies of some sort travelling around 
the sun, and by their motion preserved either from falling 
towards him (in which case the corona would quickly dis- 
appear) or from producing any pressure upon his surface, as 
an atmosphere would. 

Whatever the corona may be, it is clear that regarding it 
as a solar appendage — a conclusion which seems forced upon 
us by the evidence — it is presented to us as one of the most 
striking and imposing of all the phenomena of the solar 



THE SUN'S CORONA. 109 

system. It is a fitting crown of glory for that orb which 
sways the planets by its attraction, warms them by its fires, 
illuminates them by the splendour of its light, and pours 
forth on all of them the electric and chemic influences 
which are as necessary as light and heat for the welfare of 
their inhabitants. 



MYSTERIES OF TIME AND SPACE. 



THE SUN'S LONG STREAMERS. 

Professor Cleveland Abbe, an American astronomer and 
meteorologist, who had intended to observe the eclipse of 
the sun in July 1878 from the summit of Pike's Peak, in 
Colorado, more than 14,000 feet above the sea-level, fell ill 
after he had reached that place, and was carried down to 
the Lake House (elevation 10,000 feet), there to remain 
while the rest of his party stayed to view the eclipse from 
the summit. Probably if he had remained with them his 
observations would have differed in no very marked degree 
from those which other astronomers made on that occasion. 
He would have devoted a few seconds, perhaps, to the 
study of the sun's corona with the naked eye. He would 
probably have made some telescopic, spectroscopic, or 
polariscopic observations during the rest of the three 
minutes during which the total eclipse lasted, and possibly 
he might have noted some feature rather more effectively 
and satisfactorily than most of the other observers. But 
under the actual circumstances he could not hope thus to 
take his place among the thousands of observers who have 
noted the phenomena of total solar eclipses. He had no 
optical or other instrument. Worse than all, he is near- 
sighted ; and though he had a pair of spectacles, it was not 
quite strong enough to correct his near-sightedness. 

Yet Professor Abbe succeeded in making observations 
far exceeding in interest any which were made by the entire 
force of eclipse observers in 1874 and 1875, and fairly 
comparable in this respect with the most remarkable dis- 



THE SUN'S LONG STREAMERS. in 

coveries effected during the great eclipses of 1868, 1869, 
1870, and 1 87 1. Debarred from instrumental researches, 
unable to do what most observers of eclipses seem anxious 
to do — namely, to see everything that can be seen — he was 
compelled to restrict himself to precisely that line of ob- 
servation which we indicated eight years ago as likely to be 
most instructive. He gave his whole attention to the corona, 
and especially to its outlying and feebler portions. Studying 
the phenomena with the naked eye, or at least with only 
spectacles to aid him, he could recognise faint luminosity 
which the telescope would inevitably have concealed from 
his view. He was not hurried ; nor was he disturbed by 
the thought that such and such instruments must be at- 
tended to in turn while still totality lasted, with care also 
that in the darkness nothing should be disturbed or injured. 
As he said after the observations were completed, and as 
I pointed out in 1870, 'a glance of a few seconds will no 
more suffice to do justice to the delicate phenomena [of the 
corona] than it would suffice to enable a naturalist to draw 
the distinguishing features of a new shell or insect, or 
would enable an artist to correctly sketch in a landscape/ 

Before describing what Professor Abbe actually saw, it 
may be well to indicate first the nature of the observations 
he proposed to make, and secondly his preconceived ideas 
as to what he was likely to see, for otherwise the value of 
his observations will not be fully appreciated. 

My readers may perhaps remember that in the year 1870 
a discussion took place on the question whether the glory 
of light seen around the sun during total eclipse belongs to 
the sun or not. There were those who maintained very 
confidently the opinion that this glory is either a purely 
optical phenomenon only or else is due to the passage of 
the solar rays through our own atmosphere all round the 
place of the eclipsed sun. On the other hand, there were 
some (myself among the number) who pointed out that 
the corona must necessarily belong to the sun, since its 
features could not possibly be reconciled with any other 



H2 MYSTERIES OF TIME AND SPACE, 

theory. The greater number of astronomers seemed, how- 
ever, to form no opinion one way or the other, but to prefer 
to leave the matter to be decided by fresh evidence. For 
too many imagine that the best way of showing how greatly 
they value observations is by declining to investigate the 
full significance of observations already made. 

It will be remembered that before long the new ob- 
servations devised to settle a question which had been 
abundantly answered by observations already made proved 
unmistakably the solar nature of the corona; Photographs 
were taken during the total eclipse of December 1870, and 
in greater number during that of December 187 1. On the 
latter occasion photographic views of the corona taken at 
stations far apart agreed closely together, showing that the 
corona could not possibly be an atmospheric phenomenon. 
No one could imagine that the air above Baicull, where 
Mr. Davis (Lord Lindsay's photographer) took his views, 
could by some amazing accident produce coronal features 
resembling those produced by the air above Ootacamund, 
one station being close to the sea-shore, the other hundreds 
of miles inland and some 1 0,000 feet above the sea-level. 
On the other hand, the resemblance of the several views 
taken at either station showed that the coronal glory could 
not be due to the illumination of some matter on the hither 
side of the moon, but far outside our own atmosphere. For 
the solar rays, passing athwart the lunar disc to fall upon 
such matter, would shift rapidly in position as the moon 
moved onwards, so that the features seen at the beginning 
of total eclipse would differ markedly from those seen 
towards the end. Since the six pictures taken at Baicull 
closely resembled each other, as did the six taken at 
Ootacamund, so that all twelve views represented the same 
corona (though of course not all to the same distance from 
the sun), it was manifest that the corona then seen was a 
solar appendage. The actual distance to which the corona 
can be traced in these pictures corresponds to about 900,000 
miles. 



, THE SUISTS LONG STREAMERS. 113 

But the believers in an atmospheric corona were not 
even yet wholly satisfied. Nay, before the recent total 
eclipse one among them even went so far as to say that 
the observations and photographs of 1870 and 187 1, while 
demonstrating the solar nature of the glory immediately 
surrounding the sun, proved the long rays extending much 
farther from the sun to be non-solar phenomena. ' The 
non-solar origin of the radial structure/ said Mr. Lockyer as 
late as July 20 last, ' was conclusively established ' during 
the eclipse of December 1871. 

To say the truth, there is no possible way of interpreting 
the long rays as phenomena of our own atmosphere or of 
matter (gaseous, meteoric, or dust-like) on the hither side 
of the moon. The idea is one which mathematicians may 
casually have thrown out. Indeed, Madler and Airy, after 
the eclipse of 1860, advanced the hypothesis that the long 
rays belong to matter between us and the moon, while Sir 
John Herschel adopted in his ' Familiar Lectures ' the 
notion that these rays belong to matter at a great height in 
our own atmosphere. But it would be to misrepresent these 
eminent astronomers to assert that they ever maintained 
these views. The available evidence, analysed as any one 
of these mathematicians could have analysed it, had he 
seen fit, would have shown convincingly that the rays must 
come from matter lying far beyond the moon. Sir John 
Herschel admitted this in a letter addressed to my- 
self. Whether Airy or Madler ever examined the evidence 
closely I do not know. If they did they doubtless were 
led to the same result as Sir J. Herschel. The matter may 
be put in this way : — Since these long rays extend from the 
black disc of the moon during mid-totality, they occupy 
then a part of the sky where no sun-illuminated air lies at 
such a time ; therefore they cannot belong to our air : but 
if there were some very tenuous matter, aerial or dust-like, 
extending as far as the moon's orbit, the whole region of the 
sky athwart which these rays extend would contain matter 
of this sort under full solar illumination : no rays then 

I 



warn 



114 MYSTERIES OF TIME AND SPACE. 

would be seen, but a nearly uniform glare, which should 
become brighter and brighter as the distance from the sun's 
place increased. If we add to this that at midnight the 
whole of the sky, except a round spot some four or five 
times the diameter of the moon, would be occupied by this 
cis-lunar matter under direct solar illumination, instead of 
that illumination from behind which such matter would 
receive during total eclipse, we see that the darkness of our 
midnight sky speaks as decisively against this theory as does 
the brightness of the long rays seen during total eclipse. 

Notwithstanding the overwhelming evidence available 
to show that these rays lie far beyond the moon, Professor 
Abbe had adopted the opinion that the rays belong to the 
earth's atmosphere, or else are mere optical illusions. ' I 
had hitherto firmly believed them,' he says, ' to be either in 
the earth's atmosphere or in the observer's eyes.' ' Such 
rays,' he adds, ? were seen by members of my eclipse party 
at Sioux Falls City, Dakota, August 1869 ; but at that 
time and ever since I have doubted their existence.' It 
is manifest that he did not begin his observations with the 
preconceived idea that the rays belong to matter far more 
distant than the moon, but with a strong opinion, if not a 
strong prejudice, the other way. 

Next let us consider the actual circumstances under 
which he observed the eclipse, for they also are important 
in enabling us to estimate the value of his result. 'Having 
been somewhat hastily carried,' he says, ' from the summit 
of Pike's Peak down to the Lake House (elevation 10,000 
feet), I had by Monday noon recovered sufficiently to be 
laid on the ground upon a gentle slope facing westwards, 
where I studied the rays visible about the sun during 
totality. I had no optical or other instrument, and un- 
fortunately had only a pair of spectacles not quite sufficient 
even to correct my near-sightedness, By straining my eyes 
somewhat I was, however, able to do something. My whole 
attention was given to the rays that extended beyond 
the brilliant ring which I presume represents the true solar 



THE SUN'S LONG STREAMERS, 115 

atmosphere. I was undisturbed by any other consideration 
except to get a true presentation of these rays. ... I went 
over the region around the sun again and again — at least 
six times — leisurely during the 161 seconds of totality, and 
cannot doubt the truthfulness and fairness of my drawing 
and description. . . . Two stakes were driven down on 
either side of me ; and between them was placed a rotable 
axis, on which my drawing-board and paper were fastened. 
... By slightly tipping my drawing-board I kept the sun 
just above it, or just hidden from view, as I wished, 
while I drew in such details as I wished, and that too, as 
it seemed to me at the time, with great ease and accuracy, 
especially as to the angular position of the rays.' 

The moon or sun appeared surrounded by a narrow 
brilliant white ring, less than 140,000 miles broad. (We 
alter the technical indication of apparent breadth into the 
actual breadth in miles as likely to be more intelligible to 
most of our readers. ) This ring was as brilliant as the full 
moon. It was of uniform tint and light, continuous and 
without any break or structure visible to Professor Abbe. 
'Outside of this there was no other concentric coronal 
appearance and no external boundary ; but the immaculate 
blue black sky immediately adjoined this light, which I now 
call the true solar co- 5 

rona or atmosphere.' 
There was throughout 
plenty of light to read 
and write by, though 
very different from 
that given by the full 
moon, 

The picture which ^ 

accompanies ProfeS- Illustrating the fays seen round the eclipsed sun 

sor Abbe's description by Professor Abbe - 

in the Colorado Spring Daily Gazette is doubtless not 
intended to present with any accuracy the actual tints or 
degrees of brightness of the various features observed. The 

1 2 




u6 MYSTERIES OF TIME AND SPACE. 

shape of the streamers is shown with sufficient exactness in 
the accompanying figure. It will be understood, of course, 
that the rays numbered were seen on a dark background, 
the l immaculate blue ' of Abbe's description. 

The tapering ray marked No. i was the first seen by him. 
He says he saw it on his first glance at the corona. It then 
seemed to extend about three times the diameter of the 
sun ; but in a minute or so, as the observer's eyes became 
accustomed to the sight, he was able to trace its tapering 
end to a distance of six diameters of the sun's disc. ' Its 
sides were straight lines, its axis passing slightly below the 
sun's centre. Its light was an exceedingly faint and delicate 
white, apparently overlaid or intermingled with the blue of 
the atmosphere, I saw no striation, texture, or variation of 
light. There was no decided increase of brightness in that 
part of the ray near the sun's edge, nor in the axis of the 
beam, the delicate light continuing uniform up to the 
corona, in whose glare it was lost.' We must note here two 
points. In all probability the words ' in a minute or so ' 
are used in their colloquial sense ion presently, because the 
whole totality did not last two minutes and half, and in the 
course of that time Professor Abbe noted all the features 
of the corona six several times. Secondly, we find that 
both in the Daily News and in Nature Professor Abbe is 
described as tracing the rays to a distance of six degrees 
from the eclipsed sun, not six diameters only ; so that, as 
the sun's apparent diameter is little more than half a degree, 
these accounts would suggest that he saw the rays to double 
the distance described in the Colorado Daily Gazette. But 
there seems little reason to doubt that the accounts given in 
the Daily Nezus and Nature, which constitute in reality but 
one account, seeing that they both came from the same 
source, are incorrect ; for the account sent to the Colorado 
paper was written by Professor Abbe himself It contains 
an illustration from a drawing of his own (reproduced 
above), which agrees with his description. Moreover, we 
received the paper directly from Professor Abbe; and 



. THE SUN'S LONG STREAMERS, 117 

unquestionably he would have struck out the word 
' diameters ' and substituted ' degrees ' if he had really seen 
the ray extending to the greater distance. Note also that 
the word ' diameter' is used throughout the descriptions of 
other rays. 

The ray marked 2 was seen as soon as 1. Its bounding 
edges, diverging from each other, but not from the sun's 
centre, produced a somewhat fan-shaped ray. l When first 
seen,' says Abbe, ' I estimated its outer limit at one 
diameter, but subsequently traced it to a diameter and a 
half from the sun. Its left-hand edge appeared somewhat 
sharper and brighter than the right-hand edge. With this 
exception the light was very uniformly distributed through- 
out its surface, fading away rapidly at its outer end. It also 
remained changeless throughout the totality.' 

No. 3 was also seen at the same time as No. 1. ' It was 
narrower and shorter than No. 1 : its estimated length, three 
diameters. It broadened at its base, like No. 1, and had the 
same uniform tint and intensity.' 

No. 4 ' was not noticed at all until the totality was half 
over. Its length was one diameter, and it was certainly 
brighter at the end farthest from the sun. It remained 
perfectly steady,' adds Professor Abbe, ' after I once noticed 
it, and gradually I became aware of a faint light partially con- 
necting it with No. 3, so that the final impression left on me was 
that these two constituted one fan-shaped projection similar 
to No. 2, but fading out in the central portions. The axis 
of No. 1 and of Nos. 3 and 4 passed nearly, if not exactly, 
through the sun's centre.' 

No. 5 extended fully five diameters from the sun's limb, 
'and was in all respects similar to No. 1. Its base was 
broader than that of No. 1, which I attributed,' says Abbe, 
■ to the glare of the increasing corona ' and of a mound of 
the ruddy prominence matter (low-lying, so as to form only 
an extension of the sierra). The light of No. 5 was fainter, 
Professor Abbe thought, than that of No. 1. ' Its edges 
were straight, except in so far as the coronal glare appeared 



Ii8 MYSTERIES OF TIME AND SPACE. 

to unduly broaden the base. Its axis passed very nearly 
through the sun's centre, and was in the prolongation of 
the axis of No. 2/ 

Professor Abbe's explanation of these rays or streamers 
occurred to him an hour or so after seeing them. He 
advances it as one which ' will probably result in the over- 
throw of all previously entertained theories respecting the 
character and cause of these streams of light.' But in 
reality it is not nearly so novel as he seems to imagine. It 
is, indeed, partly new, and in my opinion it is in great part 
true ; but what is true in it is not new, and I question 
greatly whether what is new in it can possibly be true. Let 
astronomers judge. 

' Meteor streams,' says Professor Abbe, *' is the key to the 
solution — not such meteors as some suppose to be falling 
into the sun daily, but the grand streams of meteors that 
cause the numerous shooting stars of August and November, 
and of the existence of which there is indubitable proof. 
These streams consist of fine particles or pieces, each a 
long way from its neighbour, but all rushing along in 
parallel orbits, about the sun, like the falling drops of rain 
in a thunder-shower. The August stream is calculated to 
be several hundred thousand miles broad and thick, and 
many million miles long. Such a stream, when far beyond 
the sun, but still lighted up by it, would reflect to us a faint 
uniform light precisely like that of these rays. If one end 
of the stream were farther from us than the other, the effect 
of the perspective would be to produce a tapering or wedge- 
shaped appearance. In some other part of our orbit, or 
with the meteor stream in some other part of its orbit, 
the perspective might vanish and the two ends appear of 
the same width. In this way we shall undoubtedly be able 
to explain the very numerous historical and memorable 
occasions on which flaming coronas, swords, comets, &c, 
seen in the sky during a total eclipse have been regarded 
by the superstitious as Divine omens.' 

I have very little doubt that the great extension of the 



THE SUN'S LONG STREAMERS. 119 

corona in certain directions during many total eclipses, and 
the probably far greater extension of a fainter, not readily 
discerned lustre during all eclipses, is due to the existence 
of meteor streams. It is also undoubtedly true that several 
of the meteor systems encountered by our earth in her 
journey round the sun have the vast dimensions mentioned 
by Professor Abbe. Indeed, he far underrates the dimen- 
sions of the August and November meteor systems, each of 
which must be measured in length by hundreds of millions 
of miles, not by mere millions. But it is absolutely impos- 
sible that any of the meteor systems traversed by our earth, 
or any meteor systems of no greater degree of richness, 
should present the appearance of streamers surrounding 
the sun like those in our figure above. So far as the two 
systems specially mentioned by Professor Abbe are con- 
cerned, inasmuch as we know the exact shape and position 
of the orbits along which the meteors forming these systems 
travel, we can determine the exact position which the 
meteoric streams occupy in the heavens at any moment ; 
and most certainly neither of them on July 29 last occupied 
the position of the two beams shown across the sun in our 
figure. The August system was the one which at the time 
passed nearest to the sun's place on the sky, but it did not 
come within several degrees of the sun. The November 
system did not even cross the part of the sky where the sun 
was. These two systems, therefore, could not possibly be con- 
nected in any way with the two streams, of whatever nature, 
which produced the rays intersecting exactly at the sun. 

But there is a more general objection to the theory that 
such meteor systems may explain coronal streamers seen 
during total eclipses of the sun. If such streams could be 
seen when situated beyond the sun, they would be seen 
far better when opposite the sun on the dark background 
of the midnight sky. Take, for instance, the November 
meteors. We know that the flight of meteors, some 2,000 
millions of miles long, which the earth traversed in 
November 1866, 1867, 1868, 1869, 1870, and 187 1, is now 



120 MYSTERIES OF TIME AND SPACE. 

nearing the remotest part of the long orbit of the November 
system, many millions of miles beyond the path of Uranus. 
We know that at midnight in winter the richest part of that 
system lies due south, at an elevation varying from thirty to 
fifty degrees above the horizon. There, illuminated fully by 
the sun, though at a great distance from him, it ought to 
be far better seen than a similar system lying beyond the 
sun and visible only through the light of the brightest part 
of the corona. But no one has ever, on the darkest and 
clearest night and under the most favourable atmospheric 
conditions, even suspected the existence of the faintest 
possible light where the heart of the November system is 
really situated. Much less, then, could such a system be 
seen during total eclipse (if so situated as to lie athwart the 
sun). Systems less rich than the November system (the 
richest known to us) would have still less chance of being 
discerned. 

If, then, we are to account for the radial streamers seen 
by Professor Abbe, and also seen during many other total 
eclipses, though to a less distance, by the meteoric theory, 
we must consider meteor systems very unlike those through 
which the earth herself passes. The meteor systems 
required by the theory must be much denser and much 
more brightly illuminated than the August and November 
systems. To say they must be much more brightly 
illuminated is equivalent to saying that they must be much 
nearer the sun. And in this we see an escape from another 
difficulty. Meteor systems very near the sun would be far 
more likely to appear as streamers extending radially from 
him than systems at a great distance from him. A distant 
system might, by a mere chance, so appear. For instance, 
if a total eclipse of the sun had occurred on or about May 
10, 1865, the November meteor-system (whose richest part 
was then crossing the earth's track at the point she occupies 
on November 13) would have appeared, if discernible at all, 
as a streak athwart the sun's place in the sky, and there- 
fore forming two rays on opposite sides of him, somewhat 



THE SUN'S LONG STREAMERS. 121 

like 2 and 5 in our figure. Sixteen years or so earlier or 
later the November system would present a similar appear- 
ance, only very much fainter, on account of greatly increased 
distance, during a total eclipse occurring on or about 
November 13. At no other time in the year except 
November 13 and May 10, or about these dates, could the 
November system present such an appearance. But a 
system travelling close to the sun, and not far from the 
plane near which all the planets travel, would present at all 
times nearly the appearance of a pair of rays like 2 and 5 
of our figure. On this account, therefore, as well as on 
account of the greater brightness with which such meteor 
systems would be illuminated, we must prefer the theory 
that the systems to which the coronal rays are due travel 
near to the sun. 

Yet, even as thus presented, the meteor theory alone 
seems inadequate to explain the coronal streamers. There 
is an enormous mass of evidence showing that meteor 
systems are most richly strewn throughout a region around 
the sun extending nearly to the distance of the planet 
Mercury; but there is also abundant reason for believing 
that these multitudinous systems would present an appear- 
ance very different from that depicted in Professor Abbe's 
view of the coronal streamers. We want something quite 
distinct from the theory of a mere aggregation of meteors 
to account for these rays, whether pointed or fan-shaped, 
extending directly from the sun. The aggregation of me- 
teors might present the appearance of a luminous cloud 
around the place of the eclipsed sun. This cloud might be 
to some degree radiated, because each meteor system would 
have a course carrying it either directly athwart the sun's 
place on the sky, or nearly so. But there would be nothing 
like those sharply-defined streamers extending separately 
from the sun to distances of ten or twelve sun-breadths. 
Sir George Airy, describing the appearance of the corona 
during the eclipse of 1851, pictures just such a cloud as 
we should -expect to result from the aggregation of meteors. 



122 MYSTERIES OF TIME AND SPACE. 

i Its colour/ he said, ' was white, or resembling that of 
Venus j there was no flickering or unsteadiness ; it was not 
separated from the moon, nor had it any annular structure : 
it looked like a radiated luminous cloud behind the moon. 
The long streamers manifestly require a different explana- 
tion. 

I cannot but think that the true explanation of these 
streamers, whatever it may be (I am not in the least pre- 
pared to say what it is), will be found whensoever astro- 
nomers have found an explanation of comets' tails. These 
singular appendages, like the streamers seen by Professor 
Abbe, extend directly from the sun, as if he exerted some 
repellent action on the matter forming the heads of comets. 
Indeed, Sir John Herschel did not hesitate to say that the 
existence of such a repulsive force was, to all intents and 
purposes, demonstrated by the phenomena of comets' tails. 
Now we know that meteors and comets are in some way 
associated, though the actual nature of the connection 
between them is not clear. It is certain that the November 
meteors, the August meteors, and other such systems, 
follow in the track of known comets. We know that 
when, in 1862, the earth passed through the region of space 
along which Biela's comet had recently travelled, there was 
a display of thousands of meteors, all radiating from just 
that part of the heavens from which bodies travelling parallel 
to the orbit of Biela's comet would have seemed to radiate. 
It follows from this association between comets and meteors, 
and from the fact that probably thousands of meteoric and 
cometic systems travel close to the sun, that in all probability 
there must exist generally, if not always, in the sun's neigh- 
bourhood, enormous quantities of the substance whence 
comets' tails are formed by the sun's repellent action. This 
being so, we should expect to find generally, if not always, 
long streams of matter extending from the sun's immediate 
neighbourhood, in the same way that comets' tails extend 
from comets' heads. Whether the repulsive force is elec- 
trical, magnetic, or otherwise, does not at present concern 



THE SUN'S LONG STREAMERS. 123 

us, or rather it does concern us, but at present we are quite 
unable to answer the question. All that we know certainly 
is that, in the first place, the sun does in some way cause 
streams of luminous matter to appear beyond the heads of 
comets, in a direction opposite to his own, and to enormous 
distances ; and, in the second place, that the matter forming 
comets' heads is probably present at all times, in large 
quantities, in the sun's immediate neighbourhood. We 
can hence infer, with extreme probability, that such long 
streamers as Abbe saw last July, Myer in August 1869, 
Feilitsch in June i860, and several Swedish observers 
during the eclipse of 1733, are produced in the same way 
as comets' tails, and therefore really extend (as they seem 
to do) radially from the sun. It is also certain that if they 
did not extend radially from the sun, their always seeming 
to do so would be altogether inexplicable. So that the 
theory to which we are led in one direction leads us also out 
of what would else be a very perplexing difficulty in another 
direction. 

Some recent inquiries which I have made have led me 
to the belief that the radial streamers are after all meteor 
streams, opposite streamers (in the cases cited above) being 
parts of the same meteor streams lying (at the time) beyond 
the sun, so as to be seen athwart his disc. 



124 MYSTERIES OF TIME AND SPACE. 



METEORIC ASTRONOMY. 

The views respecting meteors now held by astronomers are 
of such extreme importance, whether viewed directly, or 
regarded in relation to the inferences which seem to flow 
from them, that they may be regarded as affecting our ideas 
respecting the present constitution as well as the past history 
and the future fate of all the orbs which people space. I 
propose to consider the position to which meteoric astro- 
nomy has at present been brought, and to point out the 
connection between the results now established, and the 
subject of the solar corona, which has recently occupied so 
large a share of the attention of astronomers. 

We need not consider here the history of the earlier 
meteoric theories. It would be difficult to show that the 
mere correct ideas of the Greek and some of the Roman 
writers who have spoken of meteors were less purely 
speculative than the later view that meteors are mere 
phenomena of our own air, like lightning or the aurora. 
And although the theory that meteors are bodies which 
have been expelled from lunar or planetary volcanoes was 
discussed by mathematicians of eminence, yet it was not 
based on exact observation ; so that the calculations of 
Laplace, Obbers, and others, serve rather to illustrate the 
skill of those mathematicians than to establish any conclu- 
sions of value. 

We may begin, then, by considering the first important 
fact tending to prove the extra-terrestrial nature of meteors 
and shooting-stars — the circumstance, namely, that meteoric 
displays occur commonly on certain days of the year. 



METEORIC ASTRONOMY. 125 

It had been noticed in very early times that a display of 
shooting-stars nearly always occurs on the night of August 
10. This being known in calendars as St. Lawrence's 
Day, the meteors which fall on that day have been called 
the tears of St. Lawrence. Among astronomers, however, 
they are more commonly called Perseides, for a reason 
presently to be cited. Not so early, but still many years 
before the true theory of meteors began to be recognised, 
it was known that on or about the 12th or 13th of No- 
vember shooting stars are commonly seen. When Hum- 
boldt, after witnessing the remarkable display of 1799, 
invited special attention to this circumstance, ancient re- 
cords were examined, and it was found that for several cen- 
turies this particular part of the year had been characterised 
by star-showers. 'Time out of mind/ says Sir John 
Herschel, ' those identical nights more often, but sometimes 
those immediately adjacent, have been habitually signalised 
by such exhibitions/ 

It cannot be too often insisted upon — since doubts are 
frequently expressed respecting the truth of the modern 
theory of meteors — that this circumstance of periodicity 
suffices of itself to demonstrate the extra-terrestrial nature 
of these objects. There are no meteorological phenomena 
which recur persistently on August 10 and on November 
1 3 ; terrestrial volcanoes are not then exceptionally active ; 
the moon on those dates may be in any part whatever of 
her orbit. The one circumstance to which phenomena 
recurring on a particular date can be held to point is the 
recurrent passage by the earth of a particular part of her 
orbit on that date. If we picture the earth circling around 
the sun in her wide orbit, once in each year, and remember 
that year after year as she crosses the particular point cor- 
responding to August 10, and again as she crosses the 
particular point corresponding to November 13, her air is 
alive (as it were) with meteors, we at once see that this is 
because she comes across the meteors at those stages of her 
circuit It is precisely as though a person who travelled 



126 MYSTERIES OP TIME AND SPACE. 

continually on a certain road noticed always at certain 
stages some peculiarities, such as heat or damp, or the like. 
He would be certain, after a few experiences of the sort, 
that the phenomenon was local in its nature — peculiar, in 
fact, to the particular part of the road where it was recog- 
nised. So we, who voyage with the earth on a wide path 
round the sun, must conclude — must be absolutely certain 
— that the two particular regions of circumsolar space which 
we traverse on August 10 and November 13 are swarming 
with meteors. 

Yet we cannot for a moment imagine that two clouds of 
meteors are persistently present in these two regions. Each 
meteor is as surely acted upon by the sun's mighty influence 
as this earth on which we live ; and as surely as this earth, 
if brought to rest in any way, would be attracted towards 
the sun and fall upon his globe in about 64^ days, so every 
member of a meteor-cloud placed where the August and 
November meteors are encountered would in about the 
same time fall upon the sun and be destroyed. 

It follows that the meteors must be in rapid motion, on a 
course keeping them clear of the sun's orb ; and moreover, 
that the place of those which pass away from the region 
traversed by the earth must be more or less continuously 
supplied by arriving meteors. In other words, the August 
and November meteors must form a more or less complete 
zone or ring. The degree of completeness of either ring 
must correspond to the regularity of the recurrence of star- 
falls on the dates corresponding to either system. If it 
frequently chances that the display is intermitted, either for 
a few years or for many years in succession, the inference 
will be that greater or less gaps mar the completeness of the 
meteor zone ; whereas, if no year passes without a display 
of meteors belonging to a system, we must infer as at least 
probable that the meteor system forms a complete ring. 
Thus judged, the November system appears to be very far 
from forming a continuous zone ; since the display is often 
omitted for more than twenty years in succession, and is 



METEORIC ASTRONOMY. 127 

seldom repeated during more than four or five successive 
years. The August system, on the contrary, seldom fails to 
produce a display of greater or less splendour. 

So much may be regarded as demonstrated by the evi- 
dence already referred to. But to remove any doubt which 
may remain as to the justice of the inference that the August 
and November meteors are bodies travelling in a definite 
region of interplanetary space, there remains a test of a 
decisive nature. 

Since according to the hypothesis, all the August meteors 
are travelling around the same orbital zone, they must cross 
the earth's track on paths appreciably parallel, moving also 
with equal velocities. Now, regarding the earth with reference 
only to her orbital motion, we see that every meteor of the 
August system which actually encounters the earth must 
reach the earth's globe from one and the same direction in 
space. If the earth were actually at rest, the meteors of 
this system would seem to fall like a shower coming from a 
definite quarter. This, indeed, might not appear to be the 
case to observers occupying a particular point on the earth's 
surface, simply because the meteor-shower would be only 
partially discernible by him, and the circumstances would 
be such as to cause some illusion as to its origin and nature. 
But if we regard the earth as a whole — and for a moment as 
a sentient being — it will be obvious that, supposing she were 
at rest, she would seem to be exposed to a meteoric shower, 
many meteors falling upon her and yet larger numbers 
passing by her, but all appearing to come from the same 
quarter. But taking her orbital motion into account, we 
obtain a precisely similar result, only the direction of the 
shower becomes modified. The case may be compared to 
that of a railway carriage in a steady shower of rain ; such 
a shower will appear steadily to proceed from one quarter of 
the sky whether the carriage is at rest or moving uniformly 
in a given direction. Only in the latter case, the part of the 
sky from which the shower seems to proceed will seem to be 
somewhat nearer to the point of the horizon towards which 



28 



MYSTERIES OF TIME AND SPACE. 



the train's motion is carrying the carriage. So with the earth 
passing through a meteor stream ; the meteor-shower will 
seem to fall from one and the same part of the celestial 
sphere though the earth is travelling rapidly onwards ; only 
the part of the heavens from which the shower seems to 
proceed will seem to be somewhat nearer to the point 
towards which the earth's orbital motion is carrying her 
than in the imaginary case of a fixed earth exposed to the 
continuous downfall of the meteors belonging to the August 
system. 1 But in either case the shower will seem to come 



1 In considering this matter we need not concern ourselves with 
questions of perspective, which, though commonly introduced to explain 
the subject, tend rather to perplex than 
to enlighten the learner. The case is f 

exceedingly simple in reality : — 

Thus, let E e, Fig. I, be a small 
part of the earth's orbit at the place 
where any meteor system is encoun- 
tered, and let M m indicate a portion of 
the meteor-orbit, m e' being the course 
travelled by a particular meteor while 
the earth is moving along E e', so that 
this meteor encounters the earth when 
she has arrived at e'. Then to the ob- 
server on earth unconscious of the mo- 
tion from e to e', the course on which 
the meteor seems to arrive is obviously 
in the direction of a line from M to e, 
so that a line, s e', parallel to m e is the 
seeming path of the meteor's arrival ; 
and a point s' on the celestial sphere, 
in the direction e' m' indefinitely pro- 
duced, is the point of the heavens from which that meteor seems to 
proceed. But the directions e' e' and M e' are constant and the propor- 
tion of e e' to m e' is constant ; therefore the direction e m or e' m' is 
also constant. Hence, every meteor of the system must appear to pro- 
ceed from the point s' on the celestial sphere. 

In reality every meteor of the system travels parallel to M m, and 
therefore is proceeding (at the time of crossing the earth's track) as if from 
a point s on the celestial sphere in the direction e' m indefinitely produced. 

Since the visible part of n meteor's course in ovir atmosphere is a 




Fig. i.— Illustrating the existence of 
meteoric ' iadiant points.' 



METEORIC ASTRONOMY. 129 

from a determinate point of the star-sphere enclosing the 
earth on all sides. 

Similar remarks apply to the November shower. 

Thus a test of the interplanetary nature of meteoric 
systems is at once afforded, and a test of a remarkably 
effective nature. For owing to the earth's rotation the star- 
sphere appears to rotate ; and thus the part of the heavens 
whence a meteor shower seems to proceed is continually 
changing its place with respect to the horizon. If not- 
withstanding this continual change of apparent place, a 
certain region of the stellar heavens continue to be that 
whence a meteor shower seems to come, the evidence is so 
much the stronger, because the test is applied under con- 
tinually varying conditions. 

In the case of both the meteor systems referred to, the 
test confirms the result already deduced. The meteors of 
each system seem to come from a definite region of the 
heavens, or rather from a definite point of the star-sphere. 
In the case of the August meteors this point lies towards 
the north-eastern part of the constellation Perseus, or, more 
exactly, the r.a. of the point is about 3 h. 24 m., its north 
declination about 55 . The November meteors appear to 
come from a point in the constellation Leo, between the 
stars e and 4 of that constellation, or, more exactly, the r.a. 
of the point is about 9 h. 52 m., its north declination about 
2 4 . These points are called the ' radiant points ' of the 
meteor systems, and because of the position of those points 
the August meteors are commonly called the Perseides, 
while the November meteors are called the Leonides. 

It would be necessary to enter at this point on a con- 
sideration of the appearances actually presented by the 
August and November shooting- stars during any consider- 
straight line (approximately) directed from one and the same point of 
the heavens (so far as a given meteor system is concerned), it follows 
that wherever the observer may be, that course must appear to be such 
that if produced backwards it would pass through that part of the 
heavens. 

K 



130 MYSTERIES OF TIME AND SPACE. 

able display, if it were my present purpose to give a full 
account of what is known respecting meteors ; but I wish, 
at present, to restrict the reader's attention as far as possible 
to the cosmical aspect of the subject. 

The test here considered becomes, after it has served 
this particular turn, the means of proving the existence of 
other meteor systems which produce no recurrent showers, 
or no displays so persistently recurrent as to leave us assured 
of their being due to a true meteor system. 

For, if many meteors seen on any night appear to radiate 
from a particular part of the heavens, we can infer very pro- 
bably, though not quite certainly, that those meteors form a 
single group travelling all in the same direction. We can 
infer this, because we know that the observed event would 
certainly happen if the meteors form such a group ; but we 
cannot be quite certain that one group only is in question, 
because two meteor groups crossing the earth's track at the 
same place might have different directions and velocities 
so adjusted that the members of each would seem to en- 
counter the moving earth in the same direction. The 
unlikelihood of the coincidence renders the inference that 
a single group is in question so much the more probable. 
Now if, on the same night of other years, or very nearly at 
the same date, more meteors are seen to proceed from the 
same radiant point, we conclude that not a group but a 
system is in question, since at the very spot where meteors 
were really travelling in a definite direction in one year, 
other meteors are found following in their track in later 
years. Many millions of miles must separate the first set 
from those seen later ; and the inference is, that along the 
whole range of those millions of miles there are meteors 
travelling with equal velocity in one and the same direction 
— in other words, traversing a definite orbit region. 

Applying this principle, the observers of meteors — Alex- 
ander Herschel, in England ; Professor Newton, in America ; 
Secchi, in Italy ; Heis, in Germany ; Quetelet, in Belgium ; 
and Schmidt, in Greece ; besides many others — have estab- 



METEORIC ASTRONOMY. 131 

lished the existence of nearly 200 distinct meteor systems 
having radiant points north of the equator ; while Neumayer, 
at Melbourne, and other observers, have noted many others 
having southern radiant points. It may be concluded safely 
that upwards of 300 meteor systems cross the earth's orbital 
track around the sun. Counting — as resulting probably from 
the existence of corresponding but more important systems 
— those falls of fireballs and of aerolites which have been 
observed to occur on or near certain definite days, and 
taking also into account the probability that some meteoric 
systems have hitherto escaped recognition, we may infer that 
some 400 systems of bodies traversing the interplanetary 
spaces pass close to the earth's orbit. 

It does not follow, however, be it noted, that any one of 
these systems centrally crosses the earth's path, whether we 
regard that path as the elliptic line traversed by the earth's 
centre, or as the elliptic ring traversed by the earth's globe. 
On the contrary, the chances are that the earth does not 
pass quite or even nearly through the core of any meteor 
system belonging to the solar domain. 

Now when Schiaparelli commenced his inquiries, abso- 
lutely nothing was known about the extent of any one of 
the meteor systems traversed by the earth. To take the 
August and the November systems, it was known that one 
part of each lies near the earth's orbit. It was known also 
that the August system crosses the earth's track at a con- 
siderable angle, since even though the earth's motion brings 
the radiant point down (as it were) nearer to the point to- 
wards which the earth is making on August 10 (a point 
close by the star I Arietis), the radiant is still some 37 
from the ecliptic. It was known further that the November 
system travels much nearer to the plane of the ecliptic, and 
in a retrograde course (that is, meeting the earth), because 
the radiant lies within io° of the ecliptic, and almost in the 
latitude of the point towards which the earth is travelling 
on November 13 (a point close by \p Leonis). But nothing 
whatever was known as to the orbit range of either system — 

k 2 



i 3 2 MYSTERIES OF TIME AND SPACE. 

the major axis of the mean path of these meteoric families. 
For anything that had been shown to the contrary (setting 
aside as too unsatisfactory to be trusted the estimates of the 
velocities of the meteors while traversing our atmosphere), 
the part of the systems traversed by the earth might be the 
portion farthest from the sun, and the perihelion portion 
might be quite close to his orb ; or the part traversed might 
be the perihelion portion, and the aphelion portion might 
have any distance whatever — from a range little exceeding 
the earth's mean distance to one exceeding many times 
even the distance of Uranus or Neptune. This ignorance 
as to the meteoric orbit-ranges was necessarily accompanied 
by ignorance as to the real velocity with which they cross 
the earth's track, and this in its turn involved ignorance as 
to the true inclination of either system. Given the major 
axis of either meteor orbit, the real velocity of the meteors 
at the earth's distance would be known ; and since the 
earth's motion is known in velocity and direction, the 
apparent direction of the meteor's motion being also known, 
the real direction of the meteoric motions follows at once. 
That this is so can readily be demonstrated. Thus, let e e' 
s, be taken to represent the earth's ve- 

/ S f locity in magnitude and direction, 

111 — ,Jl / / and let m m' represent in magnitude 

// the real velocity of any meteors which 

seem to come in the direction s' e ; 
\ _ then we have only to describe the 
circular arc, klm, around e', with 
m m! as a radius, to obtain e' m, the 
real direction of the meteor's mo- 
^iittaS A^outS tion. (Compare the note illustrated 
p m „ir^rocSa re !inor by R S- *•) Bu ' *^ very construe- 
tion shows us that so long as the real 
meteoric velocity, m m', is unknown, we cannot tell the real 
direction, s' e, of the meteor's motion. The apparent velo- 
city, m e, would indeed give us what we want (the point m, 
in fact) ; but a meteor appears too suddenly and vanishes 



METEORIC ASTRONOMY. 133 

too quickly for exact reliance on any estimates of the velo- 
city with which they traverse our atmosphere. 1 

Meteoric astronomy had reached this stage when Schia- 
parelli, who had already directed much attention to the 
subject, and had speculated somewhat boldly upon it, was 
led to compare the motion of the great comet of 1862 
(No. III.) with that of the August meteors. Observing 
that this comet passed close to the earth's orbit on a course 
somewhat resembling that of the August meteors (on the 
assumption that they are moving with a velocity exceeding 
the earth's), he inquired whether the agreement would be 
found yet closer if the actual velocity of the comet where it 
passes close to the earth's orbit were assumed to be that 
with which the meteors are travelling when they enter the 
earth's atmosphere. 

As this was the sole assumption made by Schiaparelli, it 
will be well to consider how far it affects the probability of 
inferences based on the coincidence actually recognised 
when this assumption has been made. 

The real velocity of the August meteors cannot possibly 
exceed twenty-seven miles per second • since any greater, 
where they cross the earth's track, would give them a hyper- 
bolic orbit, whereas it has been shown that they form a 
closed ring. Again, the real velocity cannot greatly fall 
short of the earth's, since otherwise the meteors of the sys- 
tem would appear to come from a point lying much nearer 
to that part of the heavens towards which the earth is 
travelling on August 1 1 than is actually the case. 

Thus, let ee' be a part of the earth's course on August 10, 
2' the point on the heavens towards which she is travelling 

1 In any exact investigation of meteoric motions the influence of 
our earth's attraction, as well as the effects of the earth's rotation— the 
former producing a real, the latter an apparent, change in the paths of 
meteors — must be taken into account. These circumstances do not, 
however, affect the general reasoning given above, and their effects are 
unimportant compared with those due to the circumsolar motions of the 
meteors and of the earth. 



134 



MYSTERIES OF TIME AND SPACE. 



at the moment. Let e / be a line directed towards the 
radiant point of the August meteors, which lies about 40 
from 2'. Then e' m perpendicular to e/ represents the 
least possible velocity for the meteors (where e e' represents 
the earth's velocity). Coming from the direction of a point 
s in the heavens, with this velocity they would seem to 
radiate from S, e' 2 being drawn parallel to e /, since a 




Fig. 3. — Showing how the arc on the heavens on which lies the true radiant of the 
August meteors can be determined by a simple construction. 



meteor which was at m when the earth was at e would meet 
the earth at e\ Again, if we take Em' equal to the dia- 
meter of a square of which e e' is a side (that is equal to Ek), 
this line will represent the greatest velocity a meteor crossing 
the earth's orbit can possibly have if it is travelling in a 
closed orbit. l This gives the direction s' m! e', along which 
an August meteor would actually travel, on the supposition 
that it has this maximum velocity. We may fairly assume 



1 Because if v be the velocity of a body moving in a circle at a 
distance d from the sun, it can be shown that the velocity in a parabolic 
orbit will be »J2 . v, at a distance d from the sun. The earth's orbit 
is near enough to a circle to render this relation applicable ; moreover, 
on August 10 the earth is nearly at her mean distance from the sun. 



METEORIC ASTRONOMY. 135 

that the meteors are not arriving on a course intermediate 
to m e' and e e', because then their relative velocity would 
fall much too far below the velocities estimated by Secchi, 
Newton, Herschel, and others. 

From some point on the arc s s f upon the celestial vault, 
therefore, the August meteors are certainly travelling ; and 
with some velocity intermediate to the velocities represented 
by the lines m' e' and m e', where e e' represents a velocity 
of 18^ miles per second. Moreover, the reader is not to 
suppose that the figure deals with hypothetical relations, 
and does not admit of being at once and definitely inter- 
preted. 2' represents a real point on the heavens, a point 
on the ecliptic close by the star I Arietis, towards which, 
as already mentioned, the earth is actually travelling on 
August 10. This point can at once be found on a 
celestial globe. 2, again, is a known point, the radiant 
of the August meteors (already indicated), and about 39 
from 2'. It can, of course, be found on a globe, since its 
r.a. and declination have been given. Now let the end of a 
cord be held down on a globe at the point corresponding to 
2 7 , and let the string be stretched over the globe across the 
point corresponding to 2, so as to pass beyond 2. It will 
indicate the position of the arc s' s on the celestial 
sphere. It will lie in a circular arc of which 2 2' will 
represent 39 . And if we take points on it ' corresponding 
to s and s' ; in other words, so that s' falls, as shown in 
Fig. 3, about 2 6° from 2, and s 90 from 2, we shall 
have on the celestial globe the arc s s', along which the true 
radiant of the August meteors must lie. In other words, 
the August meteors are really travelling, when they cross 
the earth's orbit, as if from a point, this point lying 
somewhere on an arc about 64 long, and having the same 
place upon the star-sphere that the string arc corresponding 
to s s' has upon the celestial globe. 

Thus far there has been no assumption whatever. More- 
over, without proceeding a step further, we can indicate a 
surprising coincidence which Schiaparelli was the first to 



136 MYSTERIES OF TIME AND SPACE. 

recognise. The great comet of 1862 did beyond all question 
cross the earth's orbit from the direction of this very arc 
s s', that is, as if travelling from a point lying somewhere on 
this arc. The elements of the comet as determined by 
Oppolzer sufficed to demonstrate this. Thus, without any 
assumption whatever, it was proved that comet and meteors 
reached the part e' of the earth's orbit along lines contained 
within one and the same sector of space, s e' s'. 

But Schiaparelli had already indicated reasons for 
believing that the velocity of the meteors was nearly equal 
to the maximum (indicated in Fig. 3 by the line m' e'). 
What he now did was to point out that, granted this 
assumption only, the point whence the meteors are directing 
their course as they cross the earth's orbit lies close to s' 
(on the side away from 2, of course), and that the point 
whence the comet was directing its course when it crossed 
the earth's track had appreciably the same position, — the 
comet's velocity, on this assumption, being identical with 
the meteor's velocity. 

So that to sum up — Assigning not/iing, it was demonstrated 
that a comet and a meteor system, crossing the earth's track at 
the same spot, have di7'ectio7is certainly so far agreeing as to 
lie in the same sector in space — that sector having an angle 
of only 64 ; and, assuming for the meteors a velocity cor- 
respondi7ig to what had already been assigned as the probable 
value of this element, the comet and meteors cross the earth's 
track on one and the same course, and with identical velocities . 

This in reality indicates the full extent of the coincidence 
recognised by Schiaparelli. We must not go further, as 
some have done, and say that the orbit ring of the meteors 
was further found to coincide perfectly with the orbit of the 
comets. Still less must we (as an additional proof of coinci- 
dence) set the elements of the orbits side by side, as though 
the agreement of the two in respect of eccentricity, inclina- 
tion, longitude of node and of perihelion, and so on, afforded 
so many additional evidences of coincidence. The fact is 
that, setting aside Schiaparelli's assumption as to the meteoric 



METEORIC ASTRONOMY. 137 

velocities, there is no agreement as respects any of these 
elements except the longitude of the node. There is a 
further coincidence not directly expressible in the ordinary 
orbit elements, and this coincidence, as indicated above, is 
sufficiently striking. It justifies, in fact, the assumption of 
equal velocities. Then, this assumption being made, and 
leading to the coincidence of the actual direction of the two 
orbits where they cross the earth's, the exact coincidence 
of the two orbits, in all respects, follows inevitably, — since 
two bodies moving with equal velocities and in the same 
direction as they cross one and the same point in inter- 
planetary space, must, by the law of gravity, follow identical 
orbits. 

Or the matter may be otherwise expressed thus : — It 
follows from a discussion of the motions of the comet of 
1862, that if the earth had been close by the place where 
her orbit is crossed by the comet when the comet actually 
traversed that place the comet's course would have seemed 
to be directed from the radiant of the August meteors. So 
much is certain. Schiaparelli inferred that the actual course 
of the comet as respects velocity and direction was identical 
with the course of the August meteors as they traverse the 
earth's orbit. We have already had occasion to consider 
the probability of such an inference, in speaking of the pro- 
bable association between all the meteors which on any 
night appear to have the same radiant. The chances are 
great against the coincidence being accidental. 

If I were here dealing with the history of meteoric 
astronomy, I should have to give a full account of the 
recognition of a corresponding resemblance between the 
motions of the November meteors and those of TempePs 
comet (No. L, 1866). In particular, it would be desirable 
to discuss the share which Professor Adams took in the 
work. I notice with regret, by the way, that in Dr. Schel- 
len's useful work on ' Spectrum Analysis,' the labours of 
Adams are left wholly unrecognised, while the comparatively 
less important researches of Schiaparelli, Oppolzer, Peters, 



138 MYSTERIES OF TIME AND SPACE, 

and Leverrier are pointedly referred to. This is not the 
place to supply the omission ; but I may remark that if we 
set aside the labours of Adams, the only circumstances in 
which the discussion of the November meteors differed 
from that of the August meteors were these : there was, first, 
a reason for assigning a particular period to the November 
meteors, in the circumstance that great displays of these 
shooting- stars occur at mean intervals of about 33^ years, 
and assigning a period amounted to assigning the velocity 
with which the meteors cross the earth's track ; and secondly, 
the comet which agrees in its motions with the November 
meteors was dealt with separately, and the agreement only 
recognised after both orbits had been independently worked 
out. But the choice of ^\ years as the period of the 
November meteors was still an assumption • and there were 
many eminent astronomers who regarded 34-33rds, or 
32-33rds of a year as the more probable period. In any 
case, assuming a period, the task of determining the orbit 
from the observed position of the radiant was mere mathe- 
matical child's-play. But Professor Adams achieved a 
much nobler work. For he proved from the observed dis- 
placement of the node of the November meteors — that is, 
of the point where the meteors cross the earth's track- 
that the period must be ^\ years or thereabouts. This was 
a task which none but a mathematician of the highest order 
could possibly have accomplished ; and even to such a 
mathematician the task was a most laborious one. I do not 
say that Schiaparelli's case was not made out without 
Adams's assistance. The chances against the observed 
agreement as a result of accident were as great in the case 
of the November meteors as in the case of the August 
meteors, and the chances against both coincidences being 
accidental were therefore overwhelming. But Adams un- 
doubtedly removed a whatever element of doubt still remained; 
and, furthermore, if Schiaparelli's theory had never been 
started, Adams's work would of itself have sufficed to estab- 
lish the true figure of the orbit on which the November 



METEORIC ASTRONOMY. 139 

meteors travel. It has seemed fitting to say thus much in 
recognition of the remarkable labours of our great English 
mathematician ; but it need hardly be said that the value 
of Schiaparelli's ingenious researches and theories is in no 
way affected by the matter here referred to. The great 
importance of Schiaparelli's work consists in the light which 
it throws on cosmical problems of extreme interest and 
difficulty. 

The fact, then, is demonstrated that two of the meteor 
systems encountered by the earth are so far associated with 
two comets as to travel on the same orbits. We may not 
unsafely infer that all the meteor systems encountered by 
the earth are in like manner associated with other comets. 
Nor is it very rash to assume that all comets are in like 
manner associated with meteor systems. Two cases may 
seem insufficient as a basis for such wide generalisations as 
these ; but it must be remembered that they are the only 
two cases we have yet been able to deal with satisfactorily. 
It should be remarked, however, that some other comets 
have passed close by the earth's orbit, and that in several 
cases meteoric radiant points have been recognised which 
correspond fairly with the assumption that those comets 
are followed by meteor trains. However, as nothing has 
yet been proved respecting the relative length of meteoric 
trains following after comets, we cannot expect that the 
track of every comet should to any great distance be thus 
followed ; and where the distance is relatively small, no 
evidence of the kind just referred to would be obtainable 
even a year or two (perhaps) after the comet had crossed the 
earth's orbit. 

And here a word or two may- be permitted on the 
question of the condition of comets freshly arriving on the 
scene of the solar system. It is assumed sometimes that 
the train of meteors already exists when the comet first 
comes within the solar domain. But, however this may be 
in other cases, there can be no question that in the August 
and November meteors, the train — at least the train we are 



140 MYSTERIES OF TIME AND SPACE. 

acquainted with — must have been formed after the comet 
had become a member of the solar family. This will be 
evident if we consider how this last-mentioned result can 
alone come about. Take the case of the November meteors,, 
the figure of whose orbit shows that the parent comet was 
forced to become a regular attendant of the sun by the dis- 
turbing influence of Uranus. Now, the circumstances under 
which a comet approaching the sun on a parabolic or 
hyperbolic orbit can be thus affected must be regarded as 
exceptional. The planet's influence must in the first place 
be very energetically exercised ; in other words, the arriving 
comet must pass very close to the planet ; for under any 
other circumstances the sun's influence so enormously out- 
vies the planet's that the figure of the cometic orbit would 
be very little affected. Moreover, the planet's attraction 
must produce an important balance of retardation. The 
planet will inevitably accelerate the comet up to a certain 
point, and afterwards will retard it : the latter influence 
must greatly exceed the former. To show how greatly the 
comet must be retarded, it is only necessary to mention 
that the actual velocity of the November meteors, when 
they cross the orbit of Uranus, is less than one-third of 
the velocity with which Uranus himself travels ; but their 
velocity at the same distance from the sun, when they were 
approaching him from some distant stellar domain, exceeded 
the velocity of Uranus in his orbit in the proportion of 
about 7 to 5. So that, roughly, their velocity at that dis- 
tance has been reduced in the proportion of more than 
21 to 5, or by | of its original value. This is a reduction 
of about 4^ miles per second. Now Uranus could barely 
impart this velocity to a body approaching him from an 
infinite distance under his sole influence, and coming as 
near to him as the nearest of his satellites ; but he could 
not impart so much additional velocity to a body approach- 
ing him (more rapidly of course) under solar influence. Now, 
the velocity a body can impart it can also take away. Hence 
the retarding action of Uranus could under no circum- 



METEORIC ASTRONOMY. 141 

stances account for the velocity of about 4J- miles per 
second, by which the motion of Tempel's comet must have 
been reduced, unless that comet passed closer to Uranus 
than his nearest satellite. Indeed, the maximum velocity 
which Uranus could impart to a body actually reaching his 
surface (and exposed to his sole influence in approaching 
from infinity) amounts only to 137 miles per second. Now 
the distance of Uranus's nearest satellite from the planet's 
surface is but about 84,000 miles, and Uranus traverses such 
a distance in less than five hours. So that the first and last 
meteors so influenced by Uranus as to be forced to part 
with a velocity of 4J miles per second out of their actual 
velocity of about 6 miles per second, were assuredly not 
separated by a distance equal to a five hours' voyage, or in 
miles (at 6 miles per second) by so much as 108,000 miles. 
We may, indeed, safely infer that the actual distance was 
much less than this. For though all the meteors along a 
distance of 108,000 miles might have their velocity suffi- 
ciently reduced, yet in this case some of the meteors would 
have their velocity too much reduced, and would thence- 
forth pursue orbits differing very markedly from the orbits 
traversed by the remaining members of the system. 

It follows, not merely as a probable inference, but I 
think as a demonstrated conclusion, that if the November 
meteors came originally into our system as a comet travel- 
ling sunward from infinity, then either that comet was very 
compact, or else Uranus captured only a small portion of 
the comet, the remaining portions moving thenceforth on 
orbits wholly different from the path of the November 
meteors. 

There is no escape from this conclusion ; for no other 
planet than Uranus can have brought about the subjection 
of this comet to solar rule. 

One is almost led to doubt the extra-planetary origin of 
the November meteor system altogether, and to entertain 
the theory that the comet which produced it was in the first 
instance expelled from Uranus. We need not conclude, if 



142 MYSTERIES OF TIME AND SPACE. 

we accept such a view, that the period named by Leverrier 
for the introduction of the comet into the solar system 
(a.d. 126) was necessarily the epoch of the expulsion of the 
comet by Uranus. The true epoch of this event may have 
been far more remote. 1 

This, at any rate, is certain, that if the comets belonging 
to our system have been introduced, as is commonly sup- 
posed, by planetary perturbing influences, then the actual 
number of arriving comets must have indefinitely exceeded 
the number captured in this way. For an arriving planet 
has a million chances of escaping for one of being cap- 
tured — so closely must it approach to one of the major 
planets if its motion is to be sufficiently controlled to cause 
it to become a permanent member of the solar system, with 
a perihelion near enough for the inhabitants of earth to 
recognise the comet, and with an aphelion not greatly 
beyond the orbit of the disturbing planet. 

But be this as it may, the connection between meteors 
and comets remains an established fact, the existence of 
many comets in the solar system is a reality, and the fact 
that the earth encounters more than a hundred meteor 
systems cannot be disputed. 

Now, if we consider what proportion of interplanetary 
space the earth really traverses, we shall begin to recognise 
one of the most surprising of the conclusions which are 
deducible from these demonstrated facts. If the earth were 
really, as she is sometimes pictured, a globe so large that in 
an ordinary picture of her orbit around the sun she would 
be presented, to scale, as a considerable sphere, there would 
be nothing very remarkable in the circumstance that on her 
course round the sun she should come into contact with 
many meteor systems. But when we are reminded that on 
so large a scale that the earth's orbit would be represented 

1 It is also possible to account for the present position of the 
November meteor system by supposing that the encounter took place 
during that remote period when Uranus (according to the nebular 
theory) occupied a much larger region of space than at present. 



METEORIC ASTRONOMY. 



143 



by a circle ten feet in diameter, the earth herself would be 
but about the 190th of an inch in diameter, so that the path 
her globe actually traverses would be represented by a 
circle 10 feet in diameter, and marked in with about as 
stout a stroke as the down strokes of the letters in this 
page, we see at once how minute a portion of sun-surround- 
ing space this ring-orbit really occupies. Remembering 
that nearly all the meteor systems encountered by the 
earth travel on orbits largely inclined to the plane of the 
ecliptic, and, therefore, cross that plane in two spots (un- 
equally distant in nearly every case from the sun), it will 
be seen how wonderful the circumstance is, that some 200 
such spots should fall on the fine circle which forms the 
earth's true track in the ecliptic. 

Fig. 4 is intended to illustrate the remarkable nature of 
this circumstance. Here the circle e x e 2 e 3 e 4 represents 
the earth's path around the sun, e being the place of the 
earth at the time of the winter solstice. If we suppose the 
width of the circular line e l e 2 e 3 e 4 reduced to about a thirtieth 
part of its present value, we may regard this circle as 
picturing the actual shape of the region traversed by the 
earth. Now we know very little as to the real extent of the 
hundreds of meteoric rings traversed by the earth ; but the 
cross section of each (regarded as cylindrical where cross- 
ing the ecliptic) may average perhaps a million or a million 
and a half of miles. According to the inclination of each 
and the actual direction of motion, the cross section with 
the ecliptic plane will be an ellipse of greater or less eccen- 
tricity, and having its longer axis in such and such a 
direction (which may be any whatever) with respect to the 
direction of the earth's orbit at the place of transit. These 
elliptical sections must, therefore, have some such arrange- 
ment as is depicted in Fig. 4. And it may be noted that, for 
my present purpose, it is a matter of no importance whether 
the cross sections of the meteor rings be exaggerated or the 
reverse : since, on the one hand, if the cross sections are 
really larger, the importance of the several meteor zones is 



144 



MYSTERIES OF TIME AND SPACE. 



enhanced, but the circumstance that any given meteor zone 
is encountered by the earth is rendered pro tanto less sur- 
prising ; while, on the other hand, if the cross sections are 
really smaller, we must infer that the number of meteoric 
zones is proportionately greater, to give the earth a reason- 




FlG. 



-Ideal view of the ecliptical cross sections ot about a fourth of the known 
meteor systems. 



able chance of encountering so many systems. As to the 
shape of the cross section, it matters little what opinion we 
form ; only if the real cross section is circular, the cross 
section on the ecliptic must have different oval shapes as 
shown in fig. 4. 



METEORIC ASTRONOMY, 145 

So far, then, as the meteoric systems which cross the 
ecliptic descendingly are concerned, we may fairly assume 
that their cross sections on the ecliptic form a scheme 
somewhat resembling what is pictured in the figure. It is 
known that southern skies are as plentiful in meteors as 
northern skies, if not more so ; hence we must infer that as 
many cross sections as are depicted in fig. 4 should be 
added (all overlapping the circle e : e 2 e 3 e 4 ) to represent the 
meteor systems which cross the ecliptic ascendingly, or from 
south to north. Moreover, an addition should be made for 
meteor systems which have hitherto escaped notice, as well 
as for a considerable number (perhaps nearly as many again 
as all yet mentioned) which, because they strike the earth's 
course on its inner or sun-illumined side, fall on the earth 
where day is in progress, and so escape recognition alto- 
gether. Add also the more sparse systems of 'heavier 
metal ' which produce fire-balls, and ' those others which 
have supplied the countless myriads of aerolites which are 
known to have fallen on the earth. I think it will be 
granted that, if all these circumstances were taken into 
account in Fig. 4, the earth's orbit, as there pictured, 
would be absolutely encumbered with the oval spots repre- 
senting the cross sections of meteoric systems. 

Now let it be remembered that each of the cross sections 
corresponds to a long stream of meteors, if not to a com- 
plete zone, the meteors travelling around orbits compared 
with which the orbit E t e 2 e 3 e 4 has utterly insignificant pro- 
portions. Imagine the numerous cross-sections of fig. 4 
replaced by as many curves carried on various arcs, even 
only across the circular space e l e 2 e 3 e 4 (that is, leaving out 
of account altogether those portions of the systems which 
lie farther away from the sun). Conceive the like done for 
about twice as many more systems corresponding to the 
orders just mentioned. This considered, it will surely begin 
to appear that the sphere of space around the sun, which 
e l E 2 e 3 e 4 will represent, is occupied in a remarkable manner 
by interlacing meteor- comet systems. 

L 



146 MYSTERIES OF TIME AND SPACE. 

But it is certain that the earth's orbit is not clustered 
round with meteoric cross sections in the peculiar manner 
depicted in fig. 4. That all those cross sections are there 
cannot be questioned, since so many northern meteor 
systems have been recognised. But outside and inside 
Ei e 2 e 3 e 4 there must be meteor cross sections which the 
earth does not traverse. To suppose otherwise is as though 
a person who had traversed a certain route in a rainstorm 
•should suppose that no rain had fallen to right or to left of 
his track. There is nothing in the earth's orbit to attract 
meteors. She herself has not the attractive energy neces- 
sary either to compel meteors to approach her track or to 
retain them against other attractive influences. It is only 
by chance, as it were, that her track lies thus through those 
special hundreds of meteor systems, — precisely as it is only 
hy chance that such and such raindrops fall upon our 
illustrative traveller. 

Hence, we have no choice but to suppose that the whole 
plane space within the circle E t e 2 e 3 e 4 , as well as an im- 
mense plane extent of space outside that circle, is as richly 
bespread by meteor systems as we have seen to be the case 
with the circular track e } e 2 e 3 e 4 — that is, much more richly 
than as pictured in fig. 4. And here, too, we recognise the 
small importance of the extent we have given to the meteor 
•cross sections in fig. 4 ; since, if the extent of each were 
smaller, we should have to cover the space within and 
without with greater numbers of these smaller cross sections, 
in order to account fairly for the fact that the earth's track 
lies across so many of them. 

But when we remember, further, the connection which 
Schiaparelli has shown to exist between meteor systems and 
comets, we find a reason for believing that not merely a 
uniform degree of meteoric richness continues inwards from 
the earth's orbit up to or near the very place of the sun, 
but that a great increase of richness takes place as the sun's 
place is approached. Mr. Dunkin, in his valuable Appendix 
to Lardner's ' Astronomy,' presents the actual densities 



METEORIC ASTRONOMY. 147 

with which the perihelion points of observed comets are 
distributed throughout sun-surrounding space. He gives 
the following table : — 



Distance from Sun 


Number of 


Relative Cubical 


Density of 


in Millions of Miles 


Perihelia 


Space , 


Perihelia 


O to 20 


8-6 5 


I 


8-65 


20 to 40 


1 1 70 


7 


1-67 


40 to 60 


20-50 


19 


1-06 


60 to 80 


17-20 


37 


0-47 


80 to IOO 


20 -8o 


61 


0'34 


100 to 120 


8-65 


9i 


O'lO 



Here it will be observed that the increase of density with 
approach towards the sun is very rapid indeed in the sun's 
immediate neighbourhood. It is represented by the ordi- 
nates of the curve c c' in fig. 5, the dotted part of the curve 




10 
I. 



Sun 



Fig. 5. — Illustrating the increase in the richness of cometic distribution with approach 
towards the sun. 

being that inferred from the shape of the part which has 
been determined by observation. Moreover, it should be 
noticed that comets having their perihelia between 40 and 
100 millions of miles from the sun are, on the whole, more 
likely to be recognised than those whose perihelia lie nearer 
to the sun. 

We may fairly infer that the law indicated here for 
comets is that which holds also for meteor systems. Now 



148 MYSTERIES OF TIME AND SPACE. 

let the following reasoning be carefully noted : — The mem- 
bers of a meteor system in travelling towards their perihelion 
open out their ranks, as it were, because travelling there 
with continually increasing speed, so that a given time dif- 
ference between the place of two meteors corresponds to a 
continually increasing distance. We overrate this opening out 
in taking it as proportional to the square of the distance 
from the sun. Now the volumes of spherical spaces around 
the sun vary as the cube of the distance within which they 
are enclosed. Hence, cceteris paribus, the richness of 
meteoric distribution around the sun, being proportional to 

— , must vary inversely as the distance. Thus, a set 

volume 

or group of meteors, which at a distance of 90 millions of 
miles (about equal to the earth's) would spread with a cer- 
tain degree of richness, would at a distance of 10 millions 
of miles be spread nine times more richly. Now, the above 
table shows that at a mean distance of 10 millions of miles 
(taking this as corresponding to the limits o and 20 
millions — an assumption very unfavourable to the argu- 
ment) we have a density of perihelion distribution repre- 
sented by 8-65 as against a density of only 0-34 at a distance 
of 90 millions of miles. Thus the density of perihelion 
distribution is about 25^ times greater at the former dis- 
tance ; and the actual mean meteoric density is about 
(9 x 2 5"2> or ) 2 3° tunes greater. Further, the illumination 
of these meteoric bodies at the lesser distance is 81 times 
greater, since this illumination varies as the square of the 
distance. Hence, under equally favourable conditions, the 
total quantity of light reflected from meteors within a given 
considerable space, at a distance of 10 millions of miles, 
exceeds that reflected from a set within an equal space at 
the earth's distance, in the proportion of 230 x 81 to 1, or 
upwards of 2,000 times. Nearer to the sun than this still 
enormous distance the quantity of reflected light must be 
vastly greater ; and if any meteors become incandescent 
owing to the great heat to which they are exposed, the total 



METEORIC ASTRONOMY. 149 

amount of light from these sun- surrounding regions must be 
yet further increased 

It should be noticed that the only assumption which has 
been made in the above reasoning is so far in accordance 
with the evidence actually obtained that any other assump- 
tion would have a considerable weight of probability against 
it. For if Schiaparelli's discovery has any cosmical im- 
portance at all — and everyone admits that it has, — it implies 
that all comets are followed by meteoric trains. 

The reader has only to turn to fig. 4, and to conceive 
the meteoric cross sections marked in all over the circular 
space Ej e 2 e 3 e 4 with that degree of increasing richness sun- 
wards which has been indicated above (remembering also 
that, as at present drawn, the figure shows less than a fourth 
the real number of cross sections overlapping the earth's 
orbit), to perceive how richly sun-surrounding space must be 
crowded with meteoric systems. For not the ecliptic plane 
alone, but every plane through the sun, must be similarly 
intersected. 

Albeit we must not forget that the meteor systems seve- 
rally are almost infinitely tenuous. It has been calculated 
by Professor Newton, of America, that even on the occasion 
of the great display of November meteors in America in 
1867, the portion of the system actually traversed by the 
earth contained only one meteor in 900,000 cubic miles of 
space — that is, in a cubic space nearly 100 miles long, 
broad, and deep ; so that, even taking into account the 
greatly increased richness close by the sun, we have not to 
deal with a real crowding of cosmical material, but, on the 
contrary, with an excessive tenuity, using this word to in- 
dicate the relation between the quantity of matter (how- 
ever distributed) and the volume of the space within which 
it exists. 

The reader will doubtless have surmised already the 
special purpose which I have had in view in the preceding 
inquiry. It seems to me that we have in meteoric pheno- 
mena, as well as in the associated phenomena of comets, 



150 MYSTERIES OF TIME AND SPACE. 

the explanation of some at least of the features presented 
by the solar corona. I cannot see how, on the one hand,, 
the irregularities of structure which the corona presents at 
great apparent distances (up to two or three solar diameters, 
for instance) can be accounted for except by the theory that 
during eclipse the complicated network of meteor systems 
becomes discernible ; nor, on the other, how the meteor 
systems can by any possibility escape recognition when a 
total solar eclipse is seen under favourable atmospheric 
conditions. 

It has been supposed that, because I have advocated 
another theory in explanation of other features of the corona, 
I have abandoned the meteoric theory which I had formerly 
advocated. It is true that, in general, to advocate a new 
theory implies that a theory formerly held has been aban- 
doned. But, in the present instance, this is not the case. 
The solar corona is a complicated phenomenon, and pre- 
sents features which are severally due to different causes. 
Its irregularities may reasonably be attributed to one cause, 
while such features as the straight radial extensions, rifts, 
and so on, may be (or rather must be) ascribed to another. 
It may be compared, in this sense, with the aurora. In ex- 
plaining the general nature of this phenomenon, we may call 
into our aid the meteoric particles continually descending, 
in an irregular manner, through the upper regions of air ; 
but in accounting for the auroral streamers we have to con- 
sider processes taking place along straight (possibly along 
radial) lines. 

It has been objected to the meteoric explanation, that 
the parts of the corona near the sun do not present the 
appearance which we should expect to recognise in meteors 
close by the sun, and are furthermore much brighter than 
we could expect even the innermost parts of the meteor 
region to appear. In this reasoning the circumstance seems 
to be overlooked that the meteor light which seems to come 
from regions close by the sun (assuming the meteoric theory 
to be true) does not come wholly, nor even chiefly, from 



METEORIC ASTRONOMY. 151 

meteors really so close to the solar orb. We look through 
a range of meteors two or three hundred millions of miles 
deep (taking into account space beyond the earth's orbit), 
and it is the combined effect of the light coming from the 
whole of this enormous range that we really recognise, — 
?iot in the corona, but in that proportion of the coronal light 
which is due to sun-illuminated meteors. The part of the 
range nearest to the sun may be the part most densely 
crowded and most brilliantly illuminated ; but its extent is 
limited compared with that of the whole range ; moreover, 
the meteors there situated turn but half-discs (of reflected 
light) towards the earth, those beyond showing a much 
larger proportion of their illuminated halves. It is worthy 
of notice, indeed, that the farther half of the range supplies 
much the larger proportion of the light, on account of the 
greater fulness of illumination, — for, in such a case as this, 
distance per se is an element which may be absolutely 
neglected. 1 

It need scarcely be pointed out that the spectroscope 
affords the best means for testing this question. If any 
considerable proportion of the corona's light is reflected 
from meteors, this portion of the light should exhibit a 
solar spectrum, though of great fain tn ess ; or, unless great 
light-gathering power were employed, a faint continuous 
spectrum would be seen. The zodiacal light, also, should 
exhibit a continuous spectrum if it represents the outer 
portion of the sun-surrounding meteor families. Until within 
the last few months the coronal light had been known to 
give a continuous spectrum as well as certain bright lines 
(or one bright line) ; and it had been stated that the zodiacal 

1 This at first sight may sound paradoxical ; but it is strictly true 
nevertheless. The question is one of the apparent brightness of certain 
regions of the heavens, not of the total quantity of light received from 
given groups of meteors. A group of bright objects so far off as to 
appear like a cloud would preserve its brightness absolutely unchanged, 
however far off the observer might remove. Its extent alone would 
diminish. 



152 MYSTERIES OF TIME AND SPACE. 

light gives a bright line spectrum. The first evidence was 
questionable ; the second seemed opposed to the meteoric 
theory. But Janssen has examined the coronal light with 
the most powerful light-gathering means yet employed, and 
he recognises the solar dark lines in its spectrum. This 
evidence is unquestionable. And Liais, in the clear skies of 
tropical South America, has examined the zodiacal light, and 
gets an infinitely faint continuous spectrum, so that what 
seemed a strong objection to the meteoric theory is re- 
moved. Let us pause, however. Liais has been charged with 
drawing an ideal picture of the corona during total eclipse 
(his drawing, by the way, singularly countenancing the 
meteoric theory). But it was ideal : how, then, shall Liais's 
evidence be trusted on any other subject? What, however, 
if it was not ideal at all, but simply characteristic, because 
Liais observed the eclipsed sun under singularly favourable 
conditions at his southern stations in 1858? This is pre- 
cisely the inference fairly deducible from (or rather the con- 
clusion forced upon us by) the evidence of the observers 
of the recent eclipse. Janssen speaks of special forms 
resembling those seen by Liais ; observer after observer 
speaks of complicated structures within the corona ; the 
photographs tell the same tale. 

In conclusion, I believe little question can exist that a 
large proportion of these phenomena which have seemed 
most perplexing, as well in the solar corona as in the 
zodiacal light, admit of being very readily explained when 
studied in the light of the now accepted theories of meteoric 
astronomy . 



53 



COMETS. 

The year (1881) which was to have seen the end of the 
world, because of planetary conjunctions and perihelion 
passages, because Mother Shipton had said so (or was said 
to have said so), and because the ascending gallery in the 
Great Pyramid is 1,882 inches long (so that the year 1882 
is to introduce a new era), has been remarkable in astro- 
nomical annals for the number of comets which have been 
seen. No less than six were numbered, two of which 
were of great brightness. Something still remaining — more, 
indeed, than we are always ready to admit — of old super- 
stitions respecting comets has led many to regard the 
coincidence as full of meaning. Others, not quite so cre- 
dulous, have supposed that though comets may not come 
in flights of half a dozen together to portend the end of 
the world, they may yet affect our weather in some way ; 
perhaps directly, as the moon is supposed to do (with 
very little reason) ; perhaps indirectly, by acting on the 
sun. To the astronomer the appearance of so many comets 
— some of them large ones — has been full of interest, 
because he hopes by the application of the new methods 
of research discovered within the last quarter of a century 
to solve some of the mysteries with which the whole subject 
is still fraught, despite a number of interesting discoveries 
which have recently been made. 

A brief inquiry into some of the facts which have been 
discovered respecting comets, and a discussion of some of 
those peculiarities which still remain among the greatest 



154 MYSTERIES OF TIME AND SPACE. 

mysteries of science, will probably prove acceptable at the 
present time, when comets attract so much interest and 
attention. 

Elsewhere in the solar system we meet with relations 
not differing greatly in kind from those presented by our 
own earth. We see a set of globular bodies revolving 
around the sun in nearly circular orbits, nearly in one plane, 
and all in the same direction ; we find that these globes 
rotate upon their axis — still in the same direction ; they 
have, apparently, atmospheres proportioned to their dimen- 
sions ; and many of them are attended upon by bodies 
resembling our own moon. And therefore, without enter- 
ing upon the vexed question of the plurality of worlds, we 
are able to pronounce that, if these globes are inhabited, 
dwellers upon them have, like us, their years, their days, 
their seasons ; a sun — rising in the east and setting in the 
west ; twilight and moonlight ; air and vapour ; winds and 
rain ; all things, in fact, as it would seem, necessary to 
their comfort and convenience. Here and there — as in 
the zone of asteroids and the rings of Saturn — we meet 
with novelty of arrangement or configuration ; but even 
then we find a stability, either of figure or motion, which 
renders such objects comparable, so to speak, with those we 
are accustomed to. 

But with comets the case is wholly different. When we 
have said that these objects obey the law of gravity, we have 
mentioned the only circumstance — as it would appear — in 
which they conform to the relations observed in terrestrial 
and planetary arrangements. And even this law — the 
widest yet revealed to man — they seem to obey half-un- 
willingly. We see the head of a comet tracing out sys- 
tematically enough its proper orbit, while the comet's tail is 
all unruly and disobedient. 

The paths followed by comets show no resemblance 
either to the planetary orbits or to each other. Here we 
see a comet travelling in a path of moderate extent and 
not very eccentric ; there another which rushes from a 



COMETS. 155 

distance of two or three thousand millions of miles, ap- 
proaches the sun with ever-increasing velocity until nearer 
to him than parts of his own corona (as seen in eclipses), 
sweeps around him with inconceivable rapidity, and makes 
off again to where the aphelion of its orbit lies far out in 
space beyond the most distant known planet, Neptune. 
Some comets travel in a direct, others in a retrograde, path ; 
a few near the plane of the earth's orbit, many in planes 
showing every variety of inclination. Some comets regu- 
larly return after intervals of a few years ; some after 
hundreds of years ; others are only seen once or twice, and 
then unaccountably vanish; and not a few show by the 
paths they follow that they have come from interstellar 
space to pay our system but a single visit, passing out 
again to traverse we know not what other systems or 
regions. 

The ancients believed comets to be of the same nature 
as meteors, or shooting stars — either in the earth's atmo- 
sphere not far above the clouds, or, at all events, much 
lower than the moon. These views are, however, much 
less ancient than the more correct views maintained by 
the Pythagoreans. Their doctrine was that comets are 
planetary objects, having long periods. of revolution. From 
whom this opinion was derived is uncertain. Like other 
opinions attributed to Pythagoras, it was doubtless ob- 
tained from Eastern philosophers ; but of what country — 
whether Egyptian, Persian, Indian, or Chaldsean — we have 
no means of learning. Apollonius, the Myndian, ascribes 
the opinion to the Chaldaeans. He says they spoke of 
comets as of travellers penetrating far into the upper (or 
most distant) celestial spaces. Seneca and Pliny held 
similar views, exhibiting in this respect, says Humboldt, 
the imitative faculty of the Romans. But the Greek philo- 
sopher preferred to look for a theory of the universe in the 
conceptions of his own brilliant and imaginative mind. As 
if to show future ages how little was likely to be achieved 
by the highest mental powers without the habit of patient 



156 MYSTERIES OF TIME AND SPACE. 

observation, he endeavoured to educe a system of philo- 
sophy from fancies, and to found it upon syllogisms. 
Aristotle, who may be considered the typical philosopher of 
the Greek school, included comets in the wide range of 
phenomena which he claimed the privilege of explaining. 
To him was due the opinion mentioned above — an opinion 
confidently maintained during the many centuries in which 
the philosophy of Aristotle held sway over men's minds. 
To him, also, was due a yet more remarkable opinion, the 
view, namely, that the Milky Way is a vast comet which 
continually reproduces itself ! Xenophanes and Theon, in 
the fifth century, adopted a rather singular view of the 
Aristotelian theory of comets, when they spoke ot these 
objects as ' travelling light-clouds.' 

To these fancies the ancients added the idea that the 
shapes of comets indicated their character as portents. 
Thus in fig. i five views of comets are shown, as an arrow- 
head, a sea monster, a sword, a lance, and in flames. 

Tycho Brahe was the first to express doubts respecting 
the views of Aristotle. From a careful series of observa- 
tions, he demonstrated that the orbits of comets are cer- 
tainly situated beyond the moon's orbit. He thought the 
orbits must be circular, for he lived at a time when none 
but circular orbits were conceded to the celestial bodies. 
Dorfel, a native of Upper Saxony, proved that the orbits 
of comets are either very elongated ovals or parabolas, and 
that the sun occupies a focus of the curve. It happens, 
singularly enough, that this discovery was effected but a 
year or two before Newton propounded the theory of 
gravitation. Newton himself examined the orbit of the 
great comet of 1680 (known as 'Newton's comet') and 
others ; and he found that they all accord with the law of 
gravity. 

But before long, Newton's friend and pupil, Halley, 
•effected a yet more remarkable discovery. In hopes of con- 
firming Newton's views by results founded on actual obser- 
vation, he collected all the records of comets which seemed 



COMETS. 



157 



entitled to confidence, and attempted — as well as his meagre 
materials would allow him — to calculate the elements of their 
orbits. In this way he computed the paths of no less than 




twenty-four. Among these, three presented a remarkable 
similarity. One appeared in 1531, and was described by 
Appian ; another appeared in 1607, and was observed by 



158 MYSTERIES OF TIME AND SPACE. 

Kepler ; the third was traced by Halley himself in 1682. 
The equality of the intervals between these epochs led to 
the suspicion that the same comet had appeared three times 
And Halley found, on searching historical records, that a 
comet appeared in 1305, another in 1380, and a third in 
1456. Combining these appearances with those mentioned 
before, he thought he had satisfactory evidence of identity. 
For he was sufficiently familiar with the results which might 
be expected to flow from the law of gravity, to be aware 
that absolute regularity of motion was not to be expected in 
a body traversing the solar system in an eccentric orbit, and 
swayed from its proper path by the attraction of such giant 
planets as Jupiter and Saturn. Indeed, it happens, singularly 
enough — one out of many remarkable coincidences in the 
history of comets — that the comet of 1380 was not Halley's 
comet, which really appeared in 1378, a date bringing in a 
yet greater discordance in the intervals than Halley had 
suspected and accounted for. With remarkable acumen — 
since no means existed in his day for anything like accurate 
computation — he not only pointed out the possible influence 
of the great planets in disturbing the comet in past revolu- 
tions, but he made a rough approach to an estimate of the 
effect that they would have on the period of its next visit. 
{ Instead of appearing in August 1757, as it would if its 
period remained unaltered, it will not appear,' he said, 'until 
the end of 1758 or the beginning of 1759, for it will be re- 
tarded by the action of Jupiter. Wherefore/ he adds, with 
a pardonable anxiety to secure the credit of his ingenious 
investigations, ' if it should return, according to our predic- 
tion, impartial posterity will not refuse to acknowledge that 
this was discovered by an Englishman.' 

As the time for the fulfilment of the prediction ap- 
proached, an intense interest was excited in the minds of 
astronomers. In 1757, Clairut, Lalande, and Madame 
Lepaute undertook the computation of the epoch at which 
the comet might be expected to return. They applied 
methods of investigation invented by Clairut himself. It 



COMETS. 159 

resulted from their laborious computations that April 13, 
1759, was fixed on for the epoch at which the comet should 
attain its closest approach to the sun, or, as it is technically 
•expressed, should pass its perihelion. But Clairut was 
careful to allow a month either way, on account of unavoid- 
able omissions in the calculation, and for the effects of un- 
known forces, ' such as the action of some planet too far off 
to be seen ' (a happy anticipation of modern discoveries). 

And now the heavens were swept diligently by all the 
telescopists of Europe, each eager to be the first to announce 
the discovery of an object whose appearance or non-appear- 
ance was to confirm or to disprove the Newtonian theory. 
It was actually discovered, however, without telescopic aid, 
by a Saxon farmer, George Palitsch, on Christmas Day, 
1758. It reached its perihelion on March 13, 1759, con- 
firming at once the accuracy of Clairut's computations, and 
the justice of his caution in assigning rather wide limits of 
error. 

It was now evident that comets travel, like the planets, 
in determined paths ; and also that the investigation of 
their motions is a subject worthy the study of the ablest 
mathematicians, and sufficient to tax their highest powers. 
An account of their labours would be out of place in such 
a treatise as the present ; but we recommend the subject to 
the notice of the astronomical student, as one of the most 
interesting chapters in the history of modern science. 

One comet, however, discovered not long after astro- 
nomy had achieved this triumph, seemed at first to teach 
a different lesson. In 1770 a comet appeared whose path 
turned out to be, not a long oval or parabola, as had been 
the case with all the orbits yet examined, but an ellipse of 
moderate extent, and not very eccentric. The orbit lay also 
much closer than usual to that thin slice of space (so to 
speak) within which the planets are observed to move. 
Lexell, who computed the path, found that the period of the 
comet was about five and a half years. Its return was care- 
fully watched for, but no one has eve r seen the comet since. The 



160 MYSTERIES OF TIME AND SPACE. 

cause of its disappearance, and also of its sudden appearance 
— for this was equally remarkable, when we remember that 
so conspicuous a comet could not have been circulating long 
in its small orbit without discovery — was carefully inquired 
into. The result was singular. On tracing back the path of 
the comet it was found that it must have passed very near to 
the great planet Jupiter. ' It had intruded/ says Herschel, 
1 an uninvited guest into his family circle — actually nearer to 
him than his fourth satellite.' Accordingly, the comet's 
path, originally a long oval, had been bent into a curve of 
less extent. Having once entered on this new path, the 
comet was free to follow it — always returning, be it noticed, 
to the point at which it had started on it — so long as Jupiter 
was not interfered with. But it happened, unfortunately for 
the stability of the comet's motions, that, after going twice 
round the new path, it again presented itself near Jupiter's 
track, when the planet (which had meanwhile gone once 
round his orbit) was not very far from the scene of his former 
encounter. He accordingly again exerted his- influence 
upon the unfortunate comet, and this time dismissed it on a 
path which will not admit of its approaching the earth near 
enough to be seen. 1 

Let us return, however, to Halley's comet. 

It so chances that the comet which was the first to show 
full obedience to the law of gravitation was one which ex- 
hibited in a very remarkable and significant manner the 
characteristics which distinguish comets from other heavenly 
bodies, and make them so mysterious to the student of 
science. At the return of Halley's comet, in 1836, all that 
had signalised the return in 1759 was repeated, but the 

1 It must be noticed, however, that Leverrier, who very carefully re- 
examined the question, was led to deny the accuracy of the results 
recorded above. Admitting that Jupiter has twice disturbed the comet, 
he thinks there is no certainty (for want of sufficiently accurate obser- 
vations) respecting either the original path of the comet, or that in 
which it is at present circulating unobserved— if, indeed, it has not been 
absorbed by Jupiter. 



COMETS. 161 

mathematical triumph was far greater. Damoiseau, Rosen- 
berger, and Pontecoulant calculated the comet's return to 
perihelion within two or three days, instead of a month, and 
the time when it passed this point of its orbit corresponded, 
within a few hours, to the mean of their several estimates. 
On the northern heavens, where it was first seen, the comet 
presented a remarkable appearance, with a long and brilliant 
tail stretching over an arc of many degrees upon the- sky. 
When it had passed from our northern skies, it was carried 
(after a short interval, during which it was lost to view in 
the sun's rays) to the southern heavens. Sir John Herschel, 
and Maclean (Astronomer Royal at the Cape), were prepared 
to receive it ; but when first observed by them it showed 
none of the features which made it so remarkable in our 
skies. It had no tail and scarcely any head. In fact, Sir 
John Herschel, in one account, says, that as first seen it could 
only be distinguished from a fixed star by its motion. The 
study of its gradual change of aspect from that time threw 
so much light on the nature of comets' tails and other ap- 
pendages (or at any rate of that particular comet's tail) that 
Sir John Herschel, not accustomed to be over-confident, 
said there could be no doubt as to the true interpretation of 
the observed phenomena. 

Most persons know that the name ' comet ' is derived from 
the word coma, or hair, and is applied to celestial objects 
which appear to have a hairy appendage. Modern astrono- 
mers do not, indeed, use the word coma in this sense, but 
draw a distinction between the coma and the tail. There 
can be no doubt, however, that the part now called the 
comet's tail was that from which these objects derived their 
name. The word cometa or cometes is not a lately-formed 
one ; but was used by Cicero, Tibullus, and other ancient 
writers, and it is worthy of notice that all the names applied 
to comets by the Romans had a reference to hairiness — 
stellce comantes, crinitce, concinnatce, they are called by 
Ovid, Pliny, and Cicero. The last term — signifying stars 
which show a curled or crisped hairiness — would not be very 

M 



1 62 MYSTERIES OF TIME AND SPACE. 

applicable, by the way, to any comets that have appeared 
in modern times. The Chinese applied to comets the 
name suz, or ' broom.' 

It might be supposed that the hairy, broom-like, or tail- 
like appendage, so commonly seen in comets, is really a 
distinctive feature of these comets. This, however, is far 
from being the case. A very large number of comets have 
no visible tails. We refer, of course, principally to telescopic 




Fig. 2.— Changes of a Comet when first seen. 

comets ; for very few comets which have been conspicuous 
to the naked eye have wanted this appendage. 

The co?na — in the modern astronomical sense — is never 
wanting. This term is applied to a misty, hazy light, sur- 
rounding on every side a small bright spot, which is termed 
the nucleus of the comet. 

When first seen in the telescope, a comet usually pre- 
sents a small round disc of hazy light, somewhat brighter 
near the centre. As the comet approaches the sun, the disc 
lengthens, and, if the comet is to be a tailed one, traces begin 
to be seen of a streakiness in the comet's light. Gradually 



-• COMETS. 163 

a tail is formed, which is turned always from the sun (fig. 2). 
The tail grows brighter and longer, and the head becomes 
developed into a coma surrounding a distinctly-marked 
nucleus. Presently the comet is lost to view through its 
near approach to the sun. But after a while it is again 
seen, sometimes wonderfully changed in aspect through the 
effects of solar heat. Some comets are brighter and more 
striking after passing their point of nearest approach to the 
sun (or perihelion) than before ; others are quite shorn of 
their splendour when they reappear. The latter was the 
case with the comet of 1835-36, as we have already seen. 
On the other hand, the comet of 1861 burst upon us in its 
full splendour after per i/ie/ion-passage. 

Some comets have more than one tail. One appeared in 
1744 which had five or six double tails, symmetrically dis- 
posed (if one can trust the pictures handed down to us) in 
the figure of a half-opened fan (fig. 3). Others have pre- 
sented a yet more peculiar appearance, having, besides a 
tail in the usual position, a second ' unconformable ' tail, at 
right angles to the first, or inclined to it at some incon- 
gruous, out-of-the-way angle — for instance, in one case, one 
hundred and sixty degrees. Sometimes the peculiarity is 
presented of a perfectly dark gap separating the tail from 
the head. More commonly a dark space is seen behind 
the head, but on each side of this space the light from the 
head is continued so as to form a bright border on each side 
of the tail. 

As a comet approaches the sun we have seen that a 
change takes place in the appearance of the coma and 
nucleus, and that in some instances a tail is generated. The 
process actually observed is generally this : in the forward 
part of the nucleus a turbulent action is seen to be in pro- 
gress, leading to the propulsion towards the sun of jets or 
streams of misty-looking matter. Sometimes a regular cap 
or envelope is seen to be projected in this manner towards 
the sun, or even a set of envelopes one within the other. 
The matter thus thrown off is not suffered to pass very far 



164 



MYSTERIES OF TIME AND SPACE. 



from the nucleus towards the sun, but is swept away, as fast 
as formed, in the contrary direction. If the funnel of a 




steam-engine were directed forwards, instead of upwards, 
then the appearance presented by the emitted steam, as the 



COMETS. i6$ 

engine rushed on (against a hurricane, suppose, to make 
the illustration more perfect) would exemplify the process 
which seems to be taking place around the front of the 
nucleus, and far behind it, as the matter formed is continu- 
ally swept away from the sun. The same sun which attracts 
the nucleus seems to repel the emitted matter with incon- 
ceivable energy. Consider for a moment what took place 
with Newton's comet in 1680-81 (fig. 4). When this comet 
was about as far off from the sun as our earth (ninety mil- 
lion miles) it began to throw out a tail. But the comet was 
going far nearer to the sun than this. Onwards it rushed 
under the powerful^ influence of the sun's attraction, until it 
had crossed the whole space of ninety million miles, making 
— almost in a straight line — for a point only one hundred 
and thirty thousand miles from the sun's surface. In four 
weeks it traversed that vast distance, and then, suddenly (in 
a few hours) sweeping half round the sun, started on its 
return journey. But note this : as it approached the sun, 
the comet had thrown out a tail continually increasing in 
length, and pointing back almost along the orbit ; then the 
comet is lost to sight for a few days, and when it is next 
seen returning rapidly from the sun, it has a tail pointing 
forwards (a tail which must be a different one, since — as 
Herschel says — 'we cannot conceive a comet's tail to be 
brandished round like a stick ') and ninety million miles in 
length. So that, whereas the comet, already moving with a 
tremendous acquired velocity, had taken four weeks in tra- 
versing a distance of ninety millions of miles under the sun's 
attraction, the matter composing the tail had been thrown 
to the same enormous distance by the sun's repulsion in 
scarcely one-tenth part of the time, possibly (for the tail was 
formed when first seen) in a few hours ! 

The comet of 1843 (fig. 5), was yet more remarkable 
for the dimensions of its tail and for its close approach to 
the sun. The tail of this comet stretched half-way across" 
the sky in March, 1843. Its real length was two hundred 
million miles at least, for the end of the tail was lost to view 



Fig. 4. — Comet of 1680 (Newton's). 



COMETS. 167 

through the excessive faintness of its light. So near did 
this comet pass to the sun, that many astronomers did not 
expect ever to see the comet again. But — after all but 
grazing the sun — sweeping round him at a distance of less 
than one-tenth of his diameter, the comet escaped and 
passed back again into space. 

When we see the tail of a comet occupying a volume 
thousands of times greater than that of the sun itself, the 
question naturally suggests itself, * How does it happen that 
so vast a body can sweep through the solar system without 
deranging the motion of every planet ? ' Conceding even 
an extreme tenuity to the substance composing so vast a 
volume, one would still expect its mass to be tremendous. 
For instance, if we supposed the whole mass of the tail of 
the comet of 1843 to consist of hydrogen gas (the lightest 
substance known to us), yet even then the mass of the tail 
would have largely exceeded that of the sun. Every planet 
would have been dragged from its orbit by so vast a mass 
passing so near. We know, on the contrary, that no such 
effects were produced. The length of our year did not 
change by a single second, showing that our earth had been 
neither hastened nor retarded in its steady motion round 
the sun. Thus we are forced to admit that the actual sub- 
stance of the comet was inconceivably rare. A jar-full of 
air would probably have outweighed hundreds of cubic 
miles of that vast appendage which blazed across our skies, 
to the terror of the ignorant and superstitious. 

The dread of the possible evils which might accrue from 
encountering a comet, will be diminished by a consideration 
of the extreme tenuity of these objects. But the feeling 
may still remain that influences, other than those due to 
mere weight or mass, might be ' exerted upon terrestrial 
races in the course of such an encounter. The subtle 
breath of some mephitic vapour might penetrate our atmo- 
sphere, and, if it did not bring immediate destruction, might 
leave dire forms of plague and pestilence to work their evil 
will upon the human race. This fear is not, perhaps, wholly 



COMETS. 16? 

unreasonable, though — as will presently appear — the posi- 
tive information we now have does not favour the supposi- 
tion that the tail, at any rate, of a comet is likely to exercise 
such destructive effects. And it is only the tails of comets 
that we have much chance of meeting. On account of 
their enormous volumes, it is not so utterly improbable that 
we should encounter them as that we should meet the com- 
paratively minute nuclei. In fact, there is reason for sup- 
posing that the earth actually did pass through the tail of 
the comet of 1861. At about the hour when it was calcu- 
lated that the encounter should have taken place, a strange 
auroral glare was seen in the atmosphere, but beyond this 
no effect was perceptible. 

From what we have already seen, it will be manifest that 
the formation of comets' tails is a process of a very marvel- 
lous nature, apparently involving forces other than those 
with which we are acquainted. The tail, ninety millions of 
miles in length, which was seen stretching from the head of 
Newton's comet nearly along the path which the retreating 
comet had to traverse (the comet thus passing away with its 
tail in front, instead of behind, as when it approached the 
sun), must, it would seem, have been formed by some force 
far more active than the force of gravity. The distance 
traversed by the comet in the last four weeks of its approach 
to the sun under gravity was no greater than that over which 
the matter of the tail, seen after the comet had circled around 
the sun, had been carried in a few hours. Yet we have no 
other evidence of any repulsive force at all being exerted by 
the sun — at least, no evidence which can be regarded as 
demonstrative — and still less have we any evidence of a 
repulsive force exceeding in energy the sun's attracting 
power. 

This difficulty, and the circumstance that a comet's tail 
lies in the direction opposite to the sun, or in the position 
which the shadow of the head would occupy, has led many, 
unfamiliar with the laws of optics, to suppose that the 
comet's tail may be simply the track of the luminous rays 



tfp MYSTERIES OF TIME: AND SPACE. 

which have passed through the comet's head. They seem 
to think that the head may act in some way to send a beam 
-of condensed light along the region opposite to the sun.- It 
.should hardly be necessary, however, to explain that no such 
beam of light could ever be seen where we see the comet's 
tail. The cases supposed to correspond with the formation 
in this way of the tail-like appendage are, in reality, of an 
entirely different kind. Thus, when we see a long beam 
extending from a bright light, we find that first the light has 
been caused to pass in that direction only (as when light is 
admitted into an otherwise darkened room through a hole) ; 
and secondly, there is matter along the course of the light 
to be illuminated. The beam is simply that long array of 
materiaF particles which the light illuminates while leaving 
the particles in neighbouring space in darkness. So under- 
stood, such a beam is seen to be utterly unlike a comet's 
tail ; for, in the first place, we know of no matter behind 
the head to be illuminated ; and in the second, we know 
that light is falling on the regions all around the apparent 
array of illuminated particles, so that these surrounding 
regions should be as brightly lit up, which is not the case. 

If any further doubt could remain as to this theory, it 
would be removed by, first, the circumstance that the tail 
of a comet is generally curved ; and, secondly, the existence 
of several tails extending from the head of one and the same 
comet. 

Professor Tyndall started a theory based on physical ex- 
periments, and otherwise in better accordance with scientific 
possibilities. Having found that certain gases, even in an 
exceedingly attenuated state, form a luminous cloud under 
the action of the electric light, he suggested; that a comet's 
tail may be a luminous cloud of this sort, formed in the 
aether of space by those rays of sunlight which have passed 
through the comet's head. The rays which, without passing- 
through the head, fall on the sether of space, would not call 
into existence this visible cloud, because their heating action 
would destroy what their chemical or actinic action by itself 



COMETS. -. 171 

would produce ; and as fast as the cloud behind the head 
came, through the comet's motion, under full solar action, 
it would be destroyed. So the tail would always be behind 
the head. 

It appeared to Professor Tyndall that the curvature of 
a comet's tail, or the existence of more tails than one, as in 
Donati's Comet (figs. 6 and 7), was not inconsistent with 
this interpretation. For he noticed that, according to the 
gas dealt with, the luminous cloud would take a longer or 
shorter time in becoming visible. And he suggested that 
when the cloud formed slowly, the tail would be curved, 
the part near the head being behind the position which the 
head had recently passed through, while the part near the 
end of the tail would be behind the regions through which 
the comet had passed much earlier. Such luminous trai s 
as were formed more quickly would account, he considered, 
for the straighter tails. He overlooked, I think, the circum- 
stance that the shape of the luminous cloud-trail would not 
in reality depend at all upon the length of time which the 
cloud might take in becoming visible. Light would pass 
with the same velocity through the different kinds of tenuous 
gas, and whether the cloud became visible at once along the 
space thus passed through, or did not become visible for 
several seconds, or minutes, or even hours, it would become 
visible at the farther end of its course only just so long after 
it had become visible at the nearer end, as light had taken 
in traversing the length of cloud so formed. This interval 
of time would be the same for the quickly-appearing as 
for the slowly-appearing luminous cloud, and there would, 
therefore, be no difference between their forms. It would 
be necessary to account in this way for the curvature of the 
larger tail in the figure, as compared with the straightness 
of the smaller tails, that the curved tail should have been 
more slowly extended from the head; whereas r the. theory 
gives the same rate of extension for both, namely, the J*ate 
at which light travels'. - : f '..--•-_,. - ,. . r'„,- r ~ r 

We seem almost forced^- by -the phenomena of ~, such - a 



172 



MYSTERIES OF TIME AND SPACE. 



comet as Donati's, to adopt the theory of the actual repulsion 
of matter from the head of the comet into the tails— matter 
repelled most swiftly forming the straighter tails, while 




matter repelled more slowly, and seemingly in greater abun- 
dance, forms the great curved tail. 

We shall proceed to consider further on the evidence 
which seems to show that, strange though this theory 



COMETS. 



173 



of material repulsion may be, it is in point of fact the 
only admissible theory. If this shall be established, we 
shall have to admit the existence of a repulsive force, whose 




action on the grosser material of planetary bodies is in- 
sensible. 

Be r ore we proceed to consider the theory by which, 
so far as can be judged at present, the phenomena of comets' 



174 MYSTERIES OF TIME AND SPACE. 

tails can be explained, it may be well that we should con- 
sider the evidence derived from other comets than those 
hitherto considered. 

In the first place we would direct special attention to 
the comet of 1811. In this comet, as may be seen from its 
picture in fig. 8, the various parts of the comet and its tail 
could be distinguished by the naked eye. There was the 
condensed part, called the nucleus, which in this case was 
apparently globular in form ; the nebulous envelope which 
surrounds the nucleus, the so-called coma ; the bright side- 
parts of the tail where it seems to be swept away from the 
coma, leaving a comparatively dark region behind the head ; 
and the tail, widening and growing fainter with distance from 
the head. No one, we think, who considers this picture will 
for a moment imagine that the comet is a mere lens, and its 
tail merely the track of light condensed by this lens along 
the region behind the head. Here, again, the hollow struc- 
ture of the tail seems indicated by the bright tracks on either 
side, though, as we shall endeavour to show later, the ex- 
ceedingly well-defined nature of the dark track behind the 
nucleus in many comets seems to force upon us a different 
interpretation of this singular and characteristic feature. 

In some respects the comet of 181 1 tells us more of 
cometic possibilities, so to speak, than any other comet that 
has ever yet been observed. Discovered on March 26, 181 1, 
this comet remained visible for a longer time than any yet 
seen, viz. for 16 months, 22 days. It had a tail 120 millions 
of miles in length, and 15 millions of miles in diameter at 
the widest part. The diameter of the nucleus was about 
127,000 miles ; that of the envelope round the head about 
643,000 miles. But what was so remarkable about this 
comet was, that it obtained this remarkable development 
without approaching the sun, as other comets have done. 
The usual rule with comets is that the nearer they approach 
to the sun, the more their head and tails are developed. 
But the least distance of the comet of 181 1 from the sun 
was little less than 100 millions of 'miles. Again, although 



COMETS. ,175 

it had so remarkable an appearance, as seen from the 
earth, the distance of that comet from us was at no time 
less than no millions of miles. Its true magnitude, there- 
fore, as Professor Kirkwood well remarks, 'has probably 
not been surpassed by that of any other comet which has 
yet been observed.' If its path had carried it nearer to the 




Fig. 8.— Comet of 181 1. 

sun, its appearance would probably have been terrible in 
the extreme. If we consider the enormous volume occupied 
l>y this comet and its' tail,. its 

Million cubic miles of head, 
Ten billion leagues of tail, 

we shall see that the phenomena we have to interpret ought 
_not to escape us because of minuteness of scale. 



176 MYSTERIES OF TIME AND SPACE, 

Next consider the great comet of 1861. This comet was 
■discovered on May 13, by Mr. John Tebbutt, jun., of New 
South Wales, and first accurately observed at the Sydney 
Observatory, on May 26. It passed northwards from the 
southern skies, and first became visible in Europe in the 
last week of June, 1861. The first recorded observations 
were made on the evening of June 30, nineteen days after 
it had passed its point of nearest approach to the sun. I 
remember well observing it on the morning of July 2, 1861. 
For some reason, I found it impossible to sleep that morn- 
ing, and getting up about three in the morning (the exact 
hour I do not remember, but it must have been very early), 
I saw in the east what looked at first like the rays of an 
aurora borealis. But presently I noticed that these rays 
proceeded (unlike those of the aurora) from a bright centre, 
which had been hidden by clouds when my observations 
began. I used at that time to keep a four-inch] telescope, 
mounted on a three-legged stand, in my bedroom. This 
I had quickly ready for action (noting that the object, 
owing to the approach of sunrise, was getting fainter every 
minute), and turning it on the comet, I drew a picture of 
the nucleus and coma so closely resembling that which 
appeared a week or two later in the ' Illustrated London 
News,' that I might have supposed my picture had been 
surreptitiously sent to the office of the * Illustrated,' had I 
not found it resting just where I had put it in my scientific 
portfolio. 

The comet appeared to the eye as shown in fig. 9. Sir 
John Herschel, who observed it at Collingwood, in Kent, 
remarked that it was far more brilliant than any comet he 
had ever seen, not even excepting those of 181 1 and 1858. 
The Padre Secchi, at Rome, found that in the clear skies 
of Italy the tail was fully n 8° in length, corresponding to 
nearly one-third more than the distance between the horizon 
and the point overhead. This comet, by the way, though 
only favourably visible for a very short time, remained within 
the range of telescopic vision much longer. Hind remarks 



COMETS. 



that the number of separate observations for the determina- 
tion of its orbit exceeded 1,150, and extended over a period of 
1 1^ months. It travelled on a course favouring observation, 
coming from remote distances south of the plane in which 







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the earth travels to the northern side of that plane — and, as 
it chanced, crossing the plane (about five-sixths of the way 
from the sun to the earth's orbit) just when the earth lay in 
the same direction from the sun, so that for a time she was 
within the bounds of the comet's tail-like appendage— and 
then travelling northwards on a path almost at right angles 

N 



178 MYSTERIES OF TIME AND SPACE. 

to the plane of the ecliptic. Thus the comet could be 
tracked on its retreat until, finally, distance concealed it from 
our view. 

Now, the tail of the comet of 1861, as seen in fig. 9, 
had something of the fan-like expansion observed in the 
tail of the comet of 1744; but what was known of the 
•comet's position at the time when this fan-like form was 
seen, explained the peculiarity, and showed the necessity of 
taking into account the position of a comet before attaching 
undue importance to the apparent figure of its tail. For the 
fan-like form seen on this occasion was a mere effect of per- 
spective. The end of the tail appeared very much wider 
than the part near the head — not that it reaUy was so, but 
simply because it was very much nearer to the observer on 
earth. When we were actually immersed in the tail, the 
part nearest to us, being all round, had, to all intents and 
purposes, an infinite extension. But even when the comet 
was beyond that position, or a few days earlier, before it had 
reached it, the end of the tail was much nearer to us than 
the comet's head, and thus appeared far more propor- 
tionately widened than was actually the case. 

Such considerations must always be taken into account 
in dealing with cometic phenomena. Comets, more than 
any other celestial objects (the Milky Way, regarded as a 
whole, being, perhaps, alone excepted), are affected in shape, 
and (apparently) even in their very nature, by position, and 
consequent fore- shortening. 

It may be well here to consider a case in which some 
active force (other than gravity), exerted by the sun, seems 
to have wrought the destruction of a comet, or at least to 
have broken up the comet into unrecognisable fragments. 

No comet ever observed has exhibited phenomena more 
remarkable than those displayed by the comet known as 
Biela's (more properly called Gambart's). I wish I could 
agree with a modern astronomer, who has said that no 
comet has thrown more light on the nature of these bodies ; 
but, in point of fact, it is only promise of light, not light 
itself, that we have obtained. 



COMETS. 



179 



Discovered in 1826, Biela's comet was presently found 
to be identical with one seen in 1772 by Montaigne, and 
again by Pons in 1805. A careful study of the observations 
showed that the comet travels round the sun in a period of 
about 6§ years, or, roughly, thrice in twenty years. Its path 
was found to approach very near to the path of our earth. 
The comet returned in 1832, when the ignorant were scared 




Fig. 10.— Biela's Comet in 1846, before its division into two. 



much as they have been recently by the threatened influence 
of the larger planets in perihelion. The comet crossed the 
earth's track several weeks before she herself came to the 
place where the two orbits approach nearest, and it is hardly 
necessary to say that the comet's passage did not injure the 
earth's roadway in any appreciable degree. 

In 1839 the comet returned, but was not seen, travelling 
across a part of the heavens only above the horizon in the 
day-time, so that the comet's light was hidden by the sun's. 



180 MYSTERIES OF TIME AND SPACE. 

It was at the next return in 1845-46 that the comet first 
attracted special attention. On that occasion, instead of 
behaving as comets usually do, Biela's, which in the first 
days of 1846 had presented the appearance shown in fig. 10, 
was found to have divided into two. There is some little 
doubt as to the time when the comet underwent division. 
Lieut. Maury reported on January 15 that he had seen the 
comet double on January 13 ; but Wichmann observed it as 
a single comet on the 16th. But Professor Challis, in his 
account of his own observations on the comets, states that 
even on January 15 the second comet might easily have been 
overlooked. M. Valz saw nothing unusual on the 18th and 
20th ; but on the 27th : ' I was struck with amazement/ he 
says, ' to find two nebulosities, separated by an interval of 

two minutes of arc, instead of one nebulosity alone 

Each head was followed by a short tail, whose direction was 
perpendicular to the line joining the two nebulosities.' 
Earlier, only the larger comet had had a tail, the appear- 
ance presented by the double comet being that shown in 
fig. 11. 

The two comets travelled along, side by side, until at 
last both passed out of view, at which time the distance 
between them amounted to about 157,000 miles. 

In 1852 both comets returned. Sir John Herschel says, 
in his ' Familiar Lectures on Scientific Subjects/ that when 
they returned, the distance between them was unchanged. 
This, however, was a mistake. The distance now amounted 
to about 1 \ millions of miles. Again they passed before 
the interested gaze of astronomers, travelling side by side, 
though rather far apart, until finally they disappeared from 
view — I say finally, for neither has ever been seen again. 

Whether the two comets returned in 1859 is doubtful. 
It is certain that, if they did, they would have been invisible, 
for the same reason that the comet was invisible when it 
returned in 1839. 

But in 1866 the double comet should have been well 
seen. It should be remembered that each return of a comet 



COMETS. 181 

of short period (like that which our correspondent Mr. F. 
Denning, of Bristol, discovered this year) gives the astrono- 
mer more perfect mastery of the comet's motions. The 
return could be predicted with sufficient accuracy in 1832 to 
cause the comet to be easily redetected. The next visible 
return might have involved a difficulty, because the comet 
had in the interval made two circuits. But that return was 





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Fig. 11. — Biela's Comet on January 15, after its division into two. 

successfully predicted. The return in 1845-46 was still 
more accurately calculated. Nor did the breaking up of 
the comet into two on that occasion interfere with the suc- 
cessful calculation of the return in 1852. The case may be 
compared to the rating of a clock, which is more satisfac- 
torily effected in a week than in a day, for the simple reason 
that any error of observation is spread in one case over seven 
times as long a period as in the other, and therefore affects 



1 82 MYSTERIES OF TIME AND SPACE. 

the estimate of any given circuit of the hands by an error 
only one-seventh as large. Just so, whatever error an astro- 
nomer might make in observing Biela's comet in, say, 1843,. 
was distributed over all the revolutions of the comet which 
had taken place since 1826 (one might almost say since 
1772), and in a correspondingly small degree affected the 
astronomer's estimates of the comet's motion during any 
single revolution. This being so, astronomers had good 
reason for believing that in 1866 Biela's comet would 
return. When the time came that it should have been 
visible, telescopes were turned towards the spot where 
it should have been seen. Night after night from that time 
its calculated track was swept with the finest telescopes in 
Europe and America. But no trace of the comet could be 
seen. ' It is now,' wrote Sir John Herschel, in February, 
1866, ' overdue. Its orbit has been recomputed, and an 
ephemeris calculated. Astronomers have been eagerly look- 
ing out for its reappearance for the last two months, when, 
according to all former experience, it ought to have been 
conspicuously visible, but without success ! giving rise to 
the strangest surmises. At all events, it seems to have 
fairly disappeared, and that without any such excuse as in 
the case of LexelPs — the preponderant attraction of some 
great planet. Can it have come into contact, or exceedingly 
close approach to some asteroid as yet undiscovered ; or, 
peradventure, plunged into and got bewildered among the 
ring of meteorolites, which astronomers more than suspect ? ' 

Be the cause what it might, the comet was not seen 
in 1866. In 1872 it was looked for even more carefully. 
Every possible contingency depending on planetary per- 
turbations was considered ; and the telescopes of astrono- 
mers swept, not only the calculated path, but to a consider- 
able distance on either side of it. No trace of the comet 
was seen, however, in 1872 any more than in 1869. So far 
as telescopic observation is concerned, Biela's comet seems 
to have come to the end of its career as a comet. 

Yet the observations of 1852 were not the last which 



COMETS. 183 

were made on this interesting object. It has been seen 
again, though not as a comet. Nay, the occasion on which 
it was seen in the way referred to was predicted, and the 
prediction fulfilled, even in details. For a full account of its 
reappearance — as a meteor stream — I refer the reader to my 
essay on Biela's Comet in ' Familiar Science Studies.' 



1 84 MYSTERIES OF TIME AND SPACE. 



COME TIC MYSTERIES. 

During the last two years several comets — some telescopic, 
others visible to the naked eye, and even conspicuous ob- 
jects in the heavens — have been observed, not only by the 
older methods, but by some which have only been available 
within recent years. It is naturally expected, therefore, by 
the general public that some new light should be thrown 
on these mysterious objects, whose phenomena still remain 
among the unexplained, seemingly the inexplicable, problems 
of the celestial depths. 

We propose to consider here what has thus been learned, 
and what also (unfortunately it is much more) remains still 
to be learned, respecting comets. But first it will be well to 
show what are the special phenomena which present them- 
selves for explanation. 

A comet apparently comes out from the remote depths 
of space in a condition of comparative calm. It appears as 
a small round nebulous object, looking like a tiny cloud of 
extreme tenuity — the idea of tenuity being suggested by the 
exceeding faintness of the comet's light. This cloud appears 
somewhat condensed towards the middle. As the comet 
draws nearer to the sun, it usually grows somewhat long in 
the direction of the sun ; and before long a portion within 
the part nearest the sun is seen to be brighter than the rest, 
and to have a more or less defined outline. This is the 
nucleus — sometimes seen as a larger dull disc of nearly uni- 
form brightness, at others as a mere bright point, not unlike 
a star. The fainter light around this is the coma, or hair, 



CO ME TIC MYSTERIES. 185 

which resembles a luminous fog round the nucleus, usually 
brighter on the side towards the sun, and on the other side 
growing fainter and fainter till it can no longer be seen. 
Later, this lengthening of the comet in directions towards 
and from the sun becomes more marked, until at length the 
comet may fairly be said to have a head directed towards 
the sun and a tail directed from him. Nucleus, coma, and 
tail may be very different in appearance in different comets, 
and in particular the tail may be more or less complicated 
in structure, being sometimes a mere straight streak, at 
others twofold, multiple, curved, with thwart streaks, and so 
forth — no two comets, in fine, having tails resembling each 
other except in general details. 

Dr. Huggins, in a rather disappointing article on comets, 
recently communicated to the Nineteenth Century, remarks 
that the nucleus, though an apparently insignificant speck, 
* is truly the heart and kernel of the whole thing — potentially 
it is the comet.' This has scarcely yet been proved, though it 
appears exceedingly probable. It is true, however, as he 
adds, that this part only of the comet conforms rigorously to 
the law of gravitation, and moves strictly in its orbit. ' If 
we could see a great comet,' he proceeds, ' during its distant 
wanderings, when it has put off the gala trappings of peri- 
helion excitement, it would appear as a very sober object, 
and consist of little more than nucleus alone.' This again 
seems probable, though it has never yet been proved, and 
the division of some comets into two or more parts, each 
having coma, nucleus, and tail of its own, shows that the 
nucleus cannot be, in every case, what Dr. Huggins seems 
here to suggest. Dr. Huggins has done well in saying 
(though scarcely with sufficient emphasis, considering how 
often the mistake is repeated) that ' though many telescopic 
comets are of extremely small mass, nucleus included — so 
small, indeed, that they are unable to perturb such small 
bodies as Jupiter's satellites — yet we should mistake greatly 
if we were to suppose that all comets are " airy nothings." 
In some large comets the nucleus may be a few hundred 



1 86 MYSTERIES OF TIME AND SPACE. 

miles in diameter, or even very much larger, and may con- 
sist of solid matter. It is not necessary to say that the 
collision of a cometary nucleus of this order with the earth 
would produce destruction on a wide scale.' 

It is even more necessary to correct the widely-spread 
misapprehension as to the relations between meteors and 
comets. We hear it stated that the nucleus of a comet is 
made up of meteoric stones (Professor P. G. Tait says — for 
unknown reasons — that they resemble * paving stones or 
even bricks ') as confidently as though the earth had at some 
time passed through the nucleus of a comet, and some of 
our streets were now paved with stones which had fallen to 
earth on such an occasion. As a matter of fact, all that has 
yet been proved is that meteoric bodies follow in the track 
(which is very different from the tail) of some known comets, 
and that probably all comets are followed by trains of 
meteors. These may have come out of the head or nucleus 
in some way as yet unexplained ; but it is by no means 
certain that they have done so, and is by many astronomers 
regarded as more than doubtful. 

The most important points to be noticed in the behaviour 
of large comets, as they approach the sun, is that usually the 
side of the coma which lies towards the sun is the scene of 
intense disturbance. Streams of luminous matter seem to 
rise continually towards the sun, attaining a certain distance 
from the head, when, assuming a cloud-like appearance, they 
seem to form an envelope around the nucleus. This enve- 
lope gradually increases its distance from the sun, growing 
fainter and larger, while within it the process is repeated, and 
a new envelope is formed. This in turn ascends from the 
nucleus, expanding as it does so, while within it a new enve- 
lope is formed. Meanwhile, the one first formed has grown 
fainter, perhaps has disappeared. But sometimes the process 
goes on so rapidly (a day or two sufficing for the formation 
of a complete new envelope) that several envelopes will be 
seen at the same time, the outermost faintest, the innermost 
most irregular in shape and most varied in brightness, while 



CO ME TIC MYSTERIES. 187 

the envelope or envelopes between are the best developed 
and most regular. 

The matter raised up in these envelopes seems to have 
undergone a certain change of character, causing it no 
longer to obey the sun's attractive influence, but to expe- 
rience a strong repulsive action from him, whereby it is 
apparently swept away with great rapidity to form the taiL 
' It flows past the nucleus,' says Dr. Huggins, ' on all sides, 
still ever expanding and shooting backwards until a tail is 
formed in a direction opposite to the sun. This tail is 
usually curved, though sometimes rays or extra tails sensibly 
straight are also seen.' The description is, however, incom- 
plete in one important respect. The matter raised from, 
the nucleus to form the envelopes may be, and probably is,, 
carried past the nucleus on all sides ; but the appearance 
presented by the tail just behind the nucleus is not exactly 
in accordance with our ideas as to what should result from 
the flowing past 'on all sides.' There is a dark space im- 
mediately behind the nucleus, that is, where the nucleus, if 
solid, would throw its shadow, if there were matter to 
receive the light all round so that the shadow could be 
seen. Now it may be thought at first that this corre- 
sponds exactly with what should be seen : when we look just 
behind the nucleus there is no light or very little ; when we 
look on either side of that dark space there is the luminous 
matter which has been driven back from the envelopes 
in front of the nucleus. But if the luminous matter flows 
past the nucleus on all sides, it must flow past the nucleus 
on the side nearest to the observer, and also on the side 
farthest away ; and it is just where the line of sight passes 
through these two regions of brightness that a dark streak 
is seen] behind the nucleus. Let the reader draw two 
concentric circles — one an inch in diameter, the other 
two inches — and let him then draw two parallel tangents 
to the inner circle on opposite sides of it. Supposing now 
the space between the two circles to represent in section the 
luminous matter which flows all round the nucleus, while the 



1S8 MYSTERIES OF TIME AND SPACE. 

surface of the inner circle represents the unilluminated part 
behind the nucleus, the two tangent lines will represent the 
lines of sight on either side of the dark region, where, 
.as we might expect, we get plenty of light ; and we can also 
understand very well why outside of that the line of sight 
through the luminous matter (or the chords to our outer 
circle), getting shorter and shorter, the light of the luminous 
streaks bounding this part of the tail gets fainter and 
fainter : but if just inside either of the two tangents, chords 
.are drawn parallel to them, crossing the inner circle, the 
parts of these chords which lie between the two circles are 
very nearly equal in length to the tangent lines themselves ; 
and -even a common diameter to both circles has, lying 
between them, two portions together equal to the radius of 
the outer. Hence, since the line of sight even across the 
middle of the space behind the nucleus, passes through a 
considerable range of luminous matter, while a line within 
but near the outskirts of that space passes through nearly 
as great a range of luminous matter as one just outside that 
space, there should be plenty of light where yet to the eye 
there seems to be something like absolute darkness. Either 
then the eye is greatly deceived, or else we must find some 
explanation of darkness existing where considerable bright- 
ness might be expected. * 

The matter which forms the tail, seems, as I have said, to 

1 Should the careful examination of satisfactory photographs seem 
to show that the darkness (almost blackness) behind the nucleus is 
an objective and not merely a subjective phenomenon, the following 
explanation would seem forced upon us. If the particles forming the 
envelopes are minute flat bodies, and if anything in the circumstances 
under which these particles are driven off into the tail causes them to 
always so arrange themselves that the planes in which they severally 
lie pass through the axis of the tail (which, if the tail is an electrical 
phenomenon, might very well happen) then we should find the region 
behind the nucleus very dark or almost black, for the particles in the 
direction of the line of sight there would be turned edgewise towards 
us, whereas those on either side or in the prolongation of the envelopes 
would turn their faces towards the observer. 



COMETIC MYSTERIES. 189 

be swept off from the envelopes raised by the sun's action 
on the nucleus. It seems as though the matter thus raised 
had undergone in some way a change of character, which 
caused it no longer to obey the law of gravity as it had 
done when forming part of the nucleus, but instead of 
yielding to the sun's attraction to submit rather to an in- 
tense repulsive action, carrying it at a much greater rate 
from the sun than, under the action of gravity— starting 
from rest and free from all perturbing influences — it could 
have been drawn towards him. Dr. Huggins thus words 
his account of what seems to happen : ' Now is seen to take 
place a change which is most puzzling — namely, these enve- 
lopes of light appear to give up their substance under the 
influence of a strong repulsive force exerted from the sun, 
and to be forced backwards.' Sir John Herschel, after his 
long and careful study of the comet of 1830 (Halley's at its 
second return) came to the conclusion that repulsive action 
exerted by the sun on the matter raised in these envelopes 
had been distinctly proved. 

Yet here, where we seem to have our first firm ground 
for hypothesis respecting these mysterious objects — comets' 
tails — we meet with stupendous difficulties. Consider, for 
instance, the phenomena presented by Newton's comet, 
That comet had traversed the last 90 millions of miles of its 
approach towards the sun in four weeks. At the end of 
that time it passed out of view for a few days, having then 
a tail 90 millions of miles, at least, in length. Four days 
passed, and it reappeared on the other side of the sun — 
having in the interval traversed nearly a semi- circle — in 
reality, of course, the perihelion end of its long oval path. 
At its reappearance, it had a tail still 90 millions of miles 
in length, but the tail with which it reappeared had, of 
course, a direction entirely different from that of the tail 
which had been seen before — the two directions were 
inclined about 160 degrees to each other. Now, as Sir John 
Herschel remarks, we cannot look on the tail of a comet 
as something whirled round like a stick, as the comet circles 



i 9 MYSTERIES OF TIME AND SPACE. 

around its perihelion sweep. The tail with which the 
comet reappeared must have been an entirely new forma- 
tion. Nor can we doubt that if the comet could have been 
watched as it swept around the sun, the changes in the tail's 
position which had been observed to the time of disappear- 
ance, would have been found to progress continuously, 
the tail passing by a uniform motion from the position it 
then had to that which it was observed to have at the time 
of reappearance. So that we may fairly suppose the tail 
with which the comet reappeared to have been formed in 
much less than the time during which the comet had been 
out of sight. Probably its farthest part had been formed in 
much less than a day, the part near the head being, of course, 
formed later. But if the matter repelled from the head 
was thus driven over a distance of 90 million miles in 
twenty-four hours, at the outside, the average velocity of its 
motion was about a thousand miles per second, or nearly 
three times as great as the greatest velocity which the sun 
can communicate by his attractive energy to matter ap- 
proaching him from without, even though such matter come 
to him from an almost infinite distance, and in a perfectly 
straight line — the conditions most favourable for giving a 
high rate of final velocity. Such velocity as the sun can thus 
give by his attractive energy is only given to matter which 
has been exposed a long time to his influence : but here, in 
the tail of the great comet of 1680, matter seems to have 
acquired almost instantaneously a velocity sufficing to carry 
it over 90,000,000 miles with an average speed three times 
as great as the sun can thus, after long effort, communicate 
by means of his attractive power ! 

The difficulty is so great that many efforts — some bold 
and daring, others positively wild in the unscientific absurdity 
of their nature — have been made to overcome it. 

Among the most ingenious of these is (or rather was, 
for I think it is no longer maintained even by its eminent 
author) Professor Tyndall's theory of a comet's tail as an 
actinic cloud, generated by the passage of the solar rays 



CO ME TIC MYSTERIES. 191 

through exceedingly tenuous matter, after those rays had 
been in part deprived of their heating power during their 
passage through the comet's head. According to this theory 
the actinic cloud cannot be formed under the heating rays, 
but so soon at the actinic rays fall on the tenuous matter 
alone, the cloud is formed, — so that all round the region in 
which would be the comet's shadow, there is no luminous 
cloud, while along that region the cloud exists. The rapidity 
with which light travels would of course make this explana- 
tion absolutely perfect in explaining cometic tails lying 
always exactly in a straight line directed from the sun, or 
with their axis so situated. But unfortunately this exceed- 
ingly rapid formation of the tail (a tail of 90 milion miles in 
length would be formed in about eight minutes) is more 
than observation requires or can explain. Professor Tyndall 
made a slight oversight in dealing with this part of his 
theory. Noticing that the actinic cloud, as he called it, is 
not formed instantly, but after a delay of a few seconds, 
in his experiments, he reasoned as though it would follow 
from this that the formation of the actinic cloud behind a 
comet's head in space might be a process extending its action 
in distance from the head at a rate considerably less than 
that at which light travels, yet still fast enough to account 
for the exceedingly rapid formation of the tail of Newton's 
comet, and of other similar tails. But a little consideration 
will show that the few seconds following the fall of light on 
the vapours dealt with by Tyndall, before the luminous cloud 
appeared, would produce no such effect as he imagined. 
The rate of formation of the tail would still be that at which 
light travels. Imagine the head at A, for the sake of argu- 
ment, and the sun's light after reaching A, passing on to 
B, C, D, E, &c, to Z, a distance say of 100,000,000 miles, 
in nine minutes. 

A...B....C....D....E Z. 

Suppose that, when the light has reached the vaporous 
matter lying at B, an interval of one full minute (much 



192 MYSTERIES OF TIME AND SPACE. 

greater than any noticed in Tyndall's experiments) occurs 
before the actinic cloud comes into view, a similar interval 
after the light has passed C before the cloud is seen there,, 
and so on, up to the time of the arrival of the light at Z. 
Professor Tyndall's reasoning implied that all the time-inter- 
vals thus occurring at B, C, D, E, &c, up to Z, had to be 
added together, to give the total time of the formation of 
the tail from A to Z ; and hence naturally a long time might 
elapse, and the head having at the end of this time reached 
a different position from that which it had occupied at the 
beginning, the divergence of the tail from the direction 
exactly opposite to the sun, and the curvature of the tail, 
would be alike readily accounted for. But what are the 
actual facts of the case ? The part of the tail formed latest 
by the supposed solar actinic action, namely, the part at Z, 
would be formed just ten minutes after the light had left 
A, and nine minutes after the part nearest to A had been 
formed (by the same light waves), for, nine minutes after 
leaving A, the light would be at Z, and a minute after such 
epoch (according to our supposition) the actinic cloud would 
be formed respectively at A and at Z. We get just the same 
interval — nine minutes — whether the actinic cloud appears 
immediately after light has traversed the vapour which 
is to form the cloud, or a minute after, or an hour after. In 
every case the tail would be formed outwards from A, at 
the rate at which light travels. This does not accord with 
the phenomena, — in fact the supposition that a tail could be 
formed at the rate at which light travels, will be found, on 
examination, to lead to many most manifest absurdities, 
which Professor Tyndall doubtless recognised when he 
sought escape from the supposition of such rapid tail forma- 
tion through the effects he attributed to the delayed appear- 
ance of the actinic cloud. 

Another theory in explanation of the rapid formation of 
such a tail as that of Newton's comet is worthy of far less 
notice. Professor Tyndall's theory was based on an interest- 
ing physical fact, which he had himself discovered, and 



COME TIC MYSTERIES. 193 

which was also manifestly akin in character to the formation 
of a comet's tail. The one to be now noticed was suggested 
to a mathematician by a rather familiar phenomenon, the 
effects of which on his imagination he seems to have been 
never able to entirely overcome, — at any rate no amount of 
evidence against the theory seems to counterbalance in his 
mind the notion once conceived that the theory might be 
true. (It is a way some theorists have.) 

Professor Tait was once looking at a part of the sky 
which seemed clear. As he looked, a long streak rapidly 
formed, which presently disappeared (if I remember his 
original description aright) almost as rapidly as it had 
formed. At any rate, the appearance of the streak was 
rapid enough to remind him of what astronomers said about 
the rapid (apparent) development of comets' tails. The 
phenomenon itself was easily explained. There had been a 
flight of seabirds, travelling after their wont in a widely ex- 
tended layer, which when he began his observations had 
been looked at somewhat aslant, so that — the distance being 
too great for the birds to be seen individually — nothing of 
the flight could be discerned at all. But it is evident that 
in such a case a very slight movement on the part of each 
bird would suffice so to shift the position of the layer in 
which they were travelling, that it would be seen edgewise, 
and then the birds, being so situated that the range of sight 
towards any part of the layer passed athwart a great number 
of them, would of course be seen, not individually but as a 
cloud, or long straight streak, a side view in fact of the layer 
in which they were travelling. Eureka ! shouted Professor 
Tait ; and presently announced to the world the marvellous 
theory that the rapid formation of comets' tails may be ac- 
counted for on the same general principle. Astronomers 
have found that along the tracks of some comets (where the 
tails never lie, by the way, but that is a detail) are countless 
millions of meteoric bodies separately undiscernible (and 
never yet discerned as a cloud — another detail) ; there- 
fore it follows that the tails of all comets are formed by 

o 



194 MYSTERIES OF TIME AND SPACE, 

movements of ' brickbats and paving stories ' in them (Pro- 
fessor Tait's own description of meteors), after the manner 
of the seabirds he saw from Arthur's Seat. Professor 
Thomson at the Edinburgh meeting of the British Associa- 
tion endorsed this theory with special reference to the value 
of the ' seabird analogy ' in explaining the phenomena of 
Newton's comet. Dr. Huggins, who, as he does not claim 
to be a mathematician (or, to speak more correctly, as his 
labours in physical research have not given him time for 
profound mathematical studies), may be more readily ex- 
cused, also speaks of this seabird theory as if it had some 
legitimate standing. * The tail, he conceives,' he says, re- 
ferring to Dr. Tait, ' to be a portion of the less dense part of 
the train illuminated by sunlight, and visible or invisible to 
us, according, not only to circumstances of density, illumi- 
nation, and nearness, but also of tactic arrangement, as of a 
flock of birds under different conditions of perspective.' Of 
course, the theory is utterly untenable — by astronomers who 
know something of the actual facts, and have enough mathe- 
matics to consider simple geometrical relations. Bodies 
moving in a plane surface like birds, if they individually 
travel in the same plane, keep its position unchanged. But 
if they move individually at an angle to that plane (as they 
occasionally do), they change its position — the surface how- 
ever in which they collectively are at. any moment, still re- 
maining plane. In such a case only could such a phenome- 
non as was observed by Professor Tait be seen. But in such 
a case the visibility of the streak formed by the flight of birds 
would last but a few minutes, for the same motion which 
had in a few minutes brought the streak into view would in 
the next few minutes take it out of view. During the short 
time that a flight is visible in this way, it has an unchanging 
position, or a scarcely changing one. If the tail of Newton's 
comet had rapidly formed and as rapidly vanished, remain- 
ing, while visible, in an almost unchanging position, the 
c seabird analogy ' might explain that particular phenomenon, 
however inadequate to explain multitudes of others. But 



CO ME TIC MYSTERIES. 195 

the phenomena to be explained are entirely different. Leav- 
ing out of the question the varying position and length of 
the tail as it approached the sun, and after it left the sun's 
neighbourhood, all of which were entirely inconsistent with 
the seabird analogy, what we are called upon to explain is 
that a visible tail ninety millions of miles in length, seen in 
position ia on one day, was seen three days later in position 
3A (having manifestly in the meanwhile passed through all 
the intermediate positions, in- 2 

eluding 2a). If Professor Tait, 
profound mathematician though 
he be, though he may ' differen- 
tiate and integrate like Harle- 
quin/ can show how any flight 
of bodies, like or unlike sea- 
birds, can accomplish such a * A 
feat as the above, appearing first 

to form a thin streak ai, and in less than four days a thin 
streak A3, each 90 millions of miles long, without some of 
them having had to travel a distance nearly equal to the line 
from 1 to 3 — or some 150 millions of miles long, instead of 
the trifling journeys he assigned them, he should take a rank 
above Newton and Laplace as a mathematician. But there 
is another feat apparently equally difficult to him, which he 
might achieve very readily with great advantage to those 
non-mathematicians among astronomers whom his name — 
well-deserved too — as a mathematician has hitherto misled, 
and with not less advantage to his own reputation : he might 
frankly admit that the idea which occurred to him while 
watching those unfortunate seabirds, had not quite the value 
which at the moment he mistakenly attached to it, and has 
since seemed to do. 

But apart from the consideration of theories such as 
those, either demonstrably untenable, though ingenious, 
like Professor Tyndall's, or altogether and obviously un- 
tenable like Professor Tait's, there are certain phenomena 
of comet's tails which force upon us the belief that they are 

o 2 



196 MYSTERIES OF TIME AND SPACE. 

phenomena of repulsion, though the repulsive action is of a 
kind not yet known to physicists. Amongst these are : — 

i. The curvature of all the cometic tails when not seen 
from a point in or near the place of their motion. 

2. The existence of more tails than one to the same 
comet, the different tails being differently curved. 

3. The phenomena of striations athwart the tail. 

It is evident that all these phenomena are such as we 
might fairly expect if a comet's tail is caused by the sun's 
repulsive action on molecules, raised by his heating action 
on the head. The matter thus swept away would resemble 
smoke, driven upwards from the funnel of a moving steamer, 
and then swept in any given direction by a steady wind ; we 
should see a curved train of such matter just as we see a 
curved streak of smoke. If the matter raised from the head 
is not all of one kind (and it is antecedently unlikely that it 
should be), there would be more than one trail of matter, if 
the sun's repulsive action were different on these different 
kinds of matter. Lastly, the striations seen athwart the 
tail, as in the well-known case of Donati's great comet, 
would be explained, either as due to the observed pulsa- 
tional manner in which the envelopes are raised (if matter 
were raised uniformly from the head there could be no 
formation of successive envelopes), or else as due to the 
carrying off into the main tail, where alone such striations 
are seen, of matter which, had it freed itself at the beginning, 
would have been swept off into the smaller tails, but being 
as it were entangled in the great outflow of matter forming 
the large tail, escapes later, and when it does, gets swept off 
at its own more rapid rate, and there forms a streak lying 
at an angle with the direction of the principal tail. 

Bredichin has shown that where there are three tails to 
a comet, their forms correspond with the theory that the 
envelopes raised from the head are principally formed of 
hydrogen, carbon, and iron. But this, which, if established, 
would be the most important physical discovery yet made 
respecting comets, seems open at present to considerable 



COME TIC MYSTERIES. 197 

doubt, though confirmation seems to be given to it, in some 
respects, by the results of spectroscopic analysis. 

To spectroscopic analysis we must in all probability 
look for such information respecting comets as may here- 
after enable us to understand their nature. On this point 
let us consider what is said by one who, if not the greatest 
living astronomical spectroscopist, is facile princeps in this 
country — Dr. W. Huggins. First, however, we must con- 
sider the past of this method of research as applied to 
comets. 

The first successful application of the spectroscope to 
comets was made by Donati in 1864 — the light of the 
comet being then divided into three bright bands, whose 
position, however, was not exactly determined. In 1866 
Dr. Huggins obtained two kinds of light from a telescopic 
comet, part of the comet's light giving a continuous spec- 
trum, probably reflected sunlight, the other a spectrum of 
three bands. In 1868 a comet was observed (Brorsen's) 
with more success. Three bands were seen in the spectrum 
of the light from the comet's head, and a comparison of 
these with measures of similar bright bands belonging to the 
spectra of various combinations of carbon, showed, or rather 
seemed to suggest, that f combinations of carbon might be 
present in the comet.' 

In conjunction with my friend, the late Dr. W. Allen Miller (says 
Dr. Huggins) I confronted directly with the spectroscope attached to 
the telescope, the comet's light with that from inductive sparks passing 
in olefiant gas. The sensible identity of the two spectra left no doubt 
of the essential oneness of the cometary stuff with the gas composed of 
carbon and hydrogen that was employed for comparison. Since that 
time (proceeds Dr. Huggins) the light from some twenty comets has 
been examined by different observers. The general close agreement 
in all cases, notwithstanding some small divergences, of the bright 
bands in the cometary light with those seen in the spectra of hydro- 
carbons, justifies us fully in ascribing the original light of these comets 
to matter which contains carbon in combination with hydrogen. 

Last year photography was applied to this spectroscopic 



198 MYSTERIES OF TIME AND SPACE. 

work. The spectrum of the brightest comet of that year 
was partly continuous, and on this continuous spectrum 
many of the well-known Fraunhofer lines could be traced. 
This made it certain that part of the comet's light was 
reflected sunlight ; though Dr. Huggins considers also that 
a part of the continuous spectrum of every comet is due to 
inherent light. On this point some doubt may be per- 
mitted. It is one thing for special bands to show them- 
selves, for some substances may become self- luminous under 
special conditions at very moderate temperatures ; it is 
quite another thing that the solid parts of a comet's sub- 
stance should become incandescent. I venture to express 
my own belief that this can scarcely happen except in the 
case of comets which approach very near to the sun. 
Besides the continuous spectrum with dark lines, the photo- 
graph showed also a spectrum of bright lines. 

These lines (says Dr. Huggins) possessed extreme interest, for 
there was certainly contained within this hieroglyphic writing some 
new information. A discussion of the position of these new lines 
showed them to be undoubtedly the same lines which appear in 
certain compounds of carbon. Not long before Professors Liveing and 
Dewar had found from their laboratory experiments that these lines 
are only present when nitrogen is also present, and that they indicate a 
nitrogen compound of carbon, namely : — cyanogen. Two other bright 
groups were also seen in the photograph, confirming the presence of 
hydrogen, carbon, and nitrogen. 

It is worthy of notice that, only a few days later, Dr.' H. 
Draper succeeded in obtaining a photograph of the same 
comet's spectrum. It appeared to him to confirm Dr. 
Huggins's statements, except only that the dark Fraunhofer 
lines were not visible — the photograph having probably 
been taken under less favourable conditions. 

So far, then, it seems clear that comets shine in part by 
reflecting sunlight, partly with light of their own ; the part 
of the cometic substance which certainly shines with its 
own light is gaseous, and this gas in most comets 'contains 
carbon, hydrogen, and nitrogen, possibly also oxygen, in the 



COMETIC MYSTERIES. 199 

form of hydrocarbons, cyanogen, and possibly oxygen com- 
pounds of carbon.' 

But the latest comet has brought with it fresh news. Its 
spectrum is not like that given by the comets we have been 
considering. The bright lines of sodium are seen in it, and 
also other bright lines and groups of lines, which have not 
yet been shown to be identical with any belonging to the 
hydrocarbon groups, but probably are so. Dr. Huggins's 
photograph shows, he considers, ' that the original light of 
the comet, which gives a continuous spectrum (he means 
that portion of the original light which does so), was too 
strong to allow of the Fraunhofer lines being recognised in 
the reflected solar light.' We demur to this as being shown, 
it may fairly be said to be suggested. The cyanogen groups 
are not seen. 

Such is Dr. Huggins's account ; but it is manifest that 
this comet underwent important changes, of w r hich — we are 
surprised to note — Dr. Huggins has taken no account. 
Thus, in April, Professors Tacchini and Vogel found simply 
a faint continuous spectrum. In May, Vogel found that 
the three bands associated with carbon were present, though 
faint, while there was no trace whatever of the sodium band. 
On the contrary, on the nights of June 4, 5, and 7, Dr. B. 
Hasselberg, of the Observatory of Pulkowa, found that the 
nucleus of the comet gave a very strong and extended con- 
tinuous spectrum, with an ' excessively strong ' bright line 
in the orange yellow, proved by micrometrical measurement 
to be identical with the D line — the well-known double 
sodium line of the solar spectrum. The observation was 
confirmed by Duner, Bredichin, and Vogel. On this Mr. 
Hind remarks, f it is necessary to conclude that, during the 
last fortnight of May, the spectrum of Wells's comet had 
changed in a manner of which the history of science fur- 
nishes no precedent.' It must, however, be remembered 
that as yet no comets have been examined under sufficiently 
favourable conditions to enable us to say whether the 
change thus observed was really exceptional, or only excep- 



200 MYSTERIES OF TIME AND SPACE, 

tional in being for the first time noted. Whenever such 
a comet as Donati's comes favourably under spectroscopic 
scrutiny, we shall probably learn something about these 
changes which will throw more light than anything yet 
discovered on the physical economy of these mysterious 
bodies. 

What, then, do we know certainly respecting comets ? 
What may we surmise with more or less probability ? And 
in what direction may we look with most hope for future 
information? We know certainly that, in whatever way 
they are formed, the sun excites intense disturbance in 
them as they approach him. Professor Stokes remarks that 
these effects, so much greater at a first view than we might 
fairly expect in the case of many of the comets observed, 
which have approached the sun no nearer than our own 
earth does, or not so near, may be accounted for by the 
circumstance that comets travel in what must be regarded 
as, to all intents and purposes, a vacuum. From Dr. 
Crooke's experiments on very high vacua we may infer that 
there is very little loss of heat, except by radiation. Thus 
the heat received by the meteoric components of a comet 
would be much greater than might otherwise be expected. 
Dr. Huggins mentions, in the same connection, the remark- 
able persistence of the bright trains of meteors in the rare 
upper air, which sometimes remain visible for three-quarters 
of an hour before the light fades, as the heat is gradually 
radiated away. ' Our reasoning on these points/ he remarks, 
in his dry way, ' would undergo considerable modification 
if we accepted the views as to the condition of interplanetary 
space and of the sun's action which have been recently sug- 
gested by Dr. Siemens in his solar theory ' — but of course 
we do not. 

Bredichin's researches, showing that three distinct cur- 
vatures in comets' tails correspond to the winnowing out 
by solar repulsive action of (i) hydrogen, (2) carbon, and 
(3) iron, seem worthy of careful study and investigation. 
It accords well with spectroscopic evidence as to the con- 



COME TIC MYSTERIES. 201 

dition of the matter raised in gaseous form from the nucleus • 
and if as yet we have had no direct spectroscopic evidence 
of the existence of iron in comets, we know that meteors 
are closely connected with comets, and that many meteors 
contain iron. Moreover, as unexpected spectroscopic evi- 
dence of the presence of the substance sodium, common in 
so many meteors, has been found in the case of one comet, 
we may fairly hope that under yet more favourable con- 
ditions, the presence of iron also may be recognised in the 
same way. 

How far electricity may be looked to for an explanation 
of cometic phenomena, is a doubtful point among astrono- 
mers and physicists. For my own part, I must confess I 
share the strong objections which many physicists have 
expressed against the mere vague suggestion that perhaps 
this is an electrical phenomenon, perhaps that other feature 
is electrical too, perhaps all or most of the phenomena of 
comets depend on electricity. It is so easy to make such 
suggestions, so difficult to obtain evidence in their favour 
having the slightest scientific value. Still I hold the elec- 
trical idea to be well worth careful study. Whatever credit 
may hereafter be given to any electrical theory of comets, 
will be solely and entirely due to those who may help to 
establish it upon a basis of sound evidence — none whatever 
to the mere suggestion, which has been made time and 
again since it was first advanced by Fontenelle. Dr. 
Huggins says that he finds there is a rapidly growing feeling 
among physicists that both the inherent light (which he 
prefers to call the self-light) of comets and the phenomena 
of their tails belong to the order of electrical phenomena. 
An American astronomer recently wrote to him, as to 
American views of the self-light of comets, ' I cannot speak 
with authority for anyone but myself ; still I think the pre- 
vailing impression amongst us is that this light is due to an 
electric, or, if I may coin the word ' (far better not) ' an 
electric-oid action of some kind.' On this Dr. Huggins 
himself remarks : — 



202 MYSTERIES OF TIME AND SPACE. 

The spectroscopic results fail to give conclusive evidence on this 
point ; still, perhaps, upon the whole, especially if we consider the 
photographs of last year, the teachings of the spectroscope are in 
favour of the view that the self-light of comets is due to electric dis- 
charges. Those who are disposed to believe that the truth lies in this 
direction, differ from each other in the precise modes in which they 
would apply the known laws of electric action to the phenomena of 
comets. Broadly, the different applications of principles of electricity 
which have been suggested, group themselves about the common idea, 
that great electrical disturbances are set up by the sun's action in con- 
nection with the vaporisation of some of the matter of the nucleus, and 
that the tail is probably matter carried away, possibly in connection 
with electric discharges, under an electrical influence of repulsion 
exerted by the sun. This view necessitates the supposition that the 
sun is strongly electrified, either negatively or positively, and further, 
that in the processes taking place in the comet, either of vaporisation 
or of some other kind, the matter thrown out by the nucleus has 
become strongly electrified in the same way as the sun — that is, nega- 
tively if the sun's electricity is negative, or positively if the sun's is 
positive. The enormous disturbances which the spectroscope shows to 
be always at work in the sun must be accompanied by electrical 
changes of equal magnitude, but we know nothing as to how far these 
are all, or the great majority of them, in one direction, so as to cause 
the sun to maintain permanently a high electrical state, whether 
positive or negative. 

Unless some such state of things exist, Sir John Her- 
scheFs statement ' That this force ' (the repulsive force 
forming the tail) 'cannot be of the nature of electric or 
magnetic forces/ must be accepted, for, as he points out, 
1 the centre of gravity of each particle, would not be affected ; 
the attraction on one of its sides would precisely equal the 
repulsion on the other.' Repulsion of the cometary matter 
could only take place if this matter, after it has been driven 
off from the nucleus and the sun, have both high electric 
potentials of the same kind. Further, it is suggested that 
luminous jets, streams, halos, and envelopes belong to the 
same order of phenomena as the aurora, the electrical brush, 
and the stratified discharges of exhausted tubes. 

All this, it will be noticed, is at present merely hypo- 
thetical. It is, however, worthy of notice that outside of 



COME TIC MYSTERIES. 203 

electricity there is nothing known to physicists which seems 
to afford even a promise of explanation so far at least as 
the grander and more striking (also the most mysterious) 
of cometic phenomena are concerned. It may well be that 
with our advancing knowledge of meteors and meteor 
systems, the spectroscopic analysis of the next few comets 
of the larger and completer types — comets like Donati's 
comet, the great comet of 181 1, and the comet of 186 1 — 
may throw unexpected light on mysteries which still remain 
among the most profound and unpromising problems pre- 
sented to modern science. 



204 MYSTERIES OF TIME AND SPACE. 



DANGERS FROM COMETS. 

The appearance of two large comets in 1881, and the dis- 
covery of several telescopic comets announced in scientific 
journals (in 1882 also large comets were seen), have led 
many to ask whether these objects may be regarded as por- 
tents, while others, not quite so ill-informed, have yet fancied 
that there may be some connection between the comets and 
the exceptionally warm weather experienced during a portion 
of the summer. I propose to consider briefly here the ideas 
commonly entertained respecting the possible influence of 
comets on terrestrial weather, touching only in passing on 
the belief, which ought long since to have died a natural 
death, that comets are sent as signs of approaching misfor- 
tunes to the human race. 

With regard to the last-mentioned superstition, I should 
in the first place notice that in former times the belief was 
natural enough. If we consider the way in which men in 
past ages regarded the heavenly bodies, we see that whether 
they considered comets to be members of the heavenly host 
or to be appearances in the upper air, they had good reasons 
for regarding them as portentous. Perceiving that the sun 
and moon, two of the seven planets of their astronomy, 
exercised very important influences on the earth, the moon 
ruling the tides and measuring the night, while the changes 
as well of the circling year with its seasons as of the day with 
its hours of morning, noon, and evening, were manifestly 
dependent on the sun's apparent motions, it was natural that 
they should regard the other planets as similarly influential, 



DANGERS FROM COMETS. 205 

though they were not equally well able to ascertain what 
special effects each planet produced. Hence arose the 
system of astrology, a system whose importance to the men 
of past ages is seldom fully appreciated. In that system 
the fixed stars found necessarily their place, so that all the 
heavenly bodies ordinarily seen — sun, moon, planets, and 
stars — were regarded as of extreme importance to the 
human race, because in their ever-varying positions those 
bodies were supposed to exert ever- varying influences. If 
comets were to be looked upon (as by the Chaldeans, whose 
doctrine was later advocated by Seneca and others) as 
heavenly bodies, moving like the planets in regular paths, it 
was natural that to them should be assigned an influence of a 
special kind, corresponding to the special character of comets 
in all respects, in their motion, in their appearance, and in 
their changes of aspect. If, on the other hand, while the 
heavenly bodies were regarded as above or in the firmament, 1 

1 I am satisfied that the doctrine of a firmament — a doctrine which 
almost all primitive or barbaric science recognises — occupies a most 
important position in the astrological beliefs with which we find it 
associated. This belief, Tylor well remarks, arises naturally in the 
minds of children, and, in accordance with the simplest childlike 
thought, the cosmologies of the North American Indians and the South 
Sea Islanders describe their flat earth arched over by the solid vault of 
heaven. Like thoughts are to be traced on through such details as the 
Zulu idea that the blue heaven is a rock encircling the earth, inside 
which are the sun, moon, and stars, and outside which dwell the 
people of heaven ; the modern negro's belief that there is a firmament 
stretched above like a cloth or web ; the Finnish poem which tells 
how Ilinarinen forged the firmament of finest steel and set in it the 
moon and stars. The New Zealander, with his notion of a solid 
firmament, through which the waters can be let down on earth through 
a crack or hole from the reservoir of rain above, could well explain the 
passage in Herodotus concerning that place in North Africa where, as 
the Libyans said, the sky is pierced, as well as the ancient Jewish 
conception of a firmament of heaven, ' strong as a molten mirror,' with 
its windows through which the rain pours down in deluge from the 
reservoirs above, windows which late Rabbinical literature tells us 
were made by taking out two stars. 



206 MYSTERIES OF TIME AND SPACE. 

the comets were regarded as below it, and, in fact, as sus- 
pended in and moving through our own air, it was natural 
that to bodies thus specially formed in a region nearer to the 
earth than that of the planets, either a more effective influ- 
ence should be assigned because of their proximity, or else 
a specially portentous character. As bodies set in or placed 
outside the firmament, the planets and fixed stars might 
have other offices, men would suppose, than to influence or 
indicate the fates and fortunes of terrestrial races ; but bodies 
specially fashioned below the firmament which separated the 
earth from the celestial regions could have no other purpose 
than to warn the human race of approaching dangers, even 
if they did not actually themselves bring the troubles — 
plagues, pestilence, famine, flood, or desolating wars — by the 
noxious influences which they spread through the environ- 
ing air. 

It was in this way no doubt that comets were originally 
regarded. They were messengers of the gods to those 
nations who believed in many gods, angels of the Lord to 
monotheistic nations. It is noteworthy, by the way, that 
neither in Assyrian tablets nor in the Bible do we find any 
reference to comets as among the heavenly bodies known 
to men in those days. This is especially remarkable when 
we consider that the writers of the tablets, as of the earlier 
books of the Bible, manifestly believed in stellar and plane- 
tary influences. In the Fifth Tablet of the Babylonian 
Creation legend we read : ' Stars, their appearance in 
figures of animals (constellations) he arranged. To fix the 
year through the observation of their constellations, twelve 
months (or signs) of stars in % - three rows he arranged, from 
the day when the year commences unto the close. He 
marked the position of the wandering stars (planets) to shine 
in their courses, that they may not do injury, and may not 
trouble anyone. . . . The God Uru (the moon) he caused 
to rise out, the night he overshadowed, to fix it also for the 
light of the night, until the shining of the day, that the 
month might not be broken, and in its amount be regular. 



DANGERS FROM COMETS. 207 

.... The God Shamas (the sun) in the horizon of the 

east to the orbit was perfected.' No word about 

comets, any more than in the corresponding description in 
the first chapter of Genesis : ' God said, Let there be lights 
in the firmament of heaven, to divide the day from the night ; 
and let them be for signs ' (their primary office in all astro- 
logical systems), ' and for seasons, and for days, and years : 
and let them be for lights in the firmament of the heaven, to 
give light upon the earth : and it was so. And God made 
two great lights ; the greater light to rule the day, and the 
lesser light to rule the night ; He made the stars also. And 
God set them in the firmament of the heaven, to give light 
upon the earth, and to rule over the day and over the night, 
and to divide the light from the darkness.' Manifestly 
comets were not regarded as among those bodies which God 
'set in the firmament of heaven.' Yet they must repeatedly 
have been seen in those times, and could not have failed to 
attract the same sort of attention then as now. It seems 
possible that there may really be a reference to comets in 
some Bible passages w T hich have been otherwise understood. 
For instance, when we remember the way in which comets 
have been compared, even in our own day, to swords 
threatening nations with punishment, it seems not unlikely 
that a comet may be referred to in 1 Chronicles xxi., verses 
14, 15, &c. : ' So the Lord sent pestilence upon Israel ; and 
there fell of Israel seventy thousand men. And God sent an 
angel unto Jerusalem to destroy it : and as he was destroy- 
ing the Lord beheld, and He repented him of the evil, and 
said to the angel that destroyed, It is enough, stay now 
thine hand. And the angel of the Lord stood by the thresh- 
ing floor of Oman the Jebusite. And David lifted up his 
eyes, and saw the angel of the Lord stand between the earth 
and the heaven, having a drawn sword in his hand stretched 
out over Jerusalem. Then David and the elders of Israel, 
who .were clothed in sackcloth, fell upon their faces. . . . 
And the Lord commanded the angel ; and he put up his 
sword again into the sheath thereof.' The whole account 



208 MYSTERIES OF TIME AND SPACE. 

from verse T4 to the end of the chapter (the last sixteen 
verses) is worth studying in this connection. Compare with 
it the following passage from Defoe's ' Plague of London : ' — 
'In the first place a blazing star or comet appeared for 
several months before the plague, as there did the year after, 
another, a little before the fire. The old women and the 
phlegmatic hypochondriacal part of the other sex, whom I 
could almost call old women too, remarked especially after- 
wards, though not till both those judgments were over, that 
those two comets passed directly over the city, and that so 
very near the houses, that it was plain they imported some- 
thing peculiar to the city alone ; and the comet before the 
pestilence was of a faint, dull, languid colour, and its motion 
very heavy, solemn, and slow ; but that the comet before the 
fire was bright and sparkling; or, as others said, flaming, 
and its motion swift and furious, and that accordingly one 
foretold a heavy judgment, but slow and severe, terrible 
and frightful, as was the plague ; but the other foretold a 
stroke sudden, swift, and fiery, as was the conflagration. 
Nay, so particular some people were, that as they looked 
upon that comet preceding the fire, they fancied that they 
not only saw it pass swiftly and fiercely, and could perceive 
the motion with their eye, but even they heard it, that it 
made a rushing mighty noise, fierce and terrible, though at a 
distance, and but just perceivable. I saw both these stars, 
and I must confess had had so much of the common notion 
of such things in my head, that I was apt to look upon them 
as the forerunners and warnings of God's judgments, and 
especially, when the plague had followed the first, I yet saw 
another of the like kind, I could not but say, God had not 
yet sufficiently scourged the city.' l 

1 Defoe adds some instructive remarks indicating the tendency of 
men at times of great trouble to be oppressed by superstitious terrors : 
' The apprehensions of the people, ' he says, ' were likewise strangely 
increased by the error of the times, in which I think the people, from 
what principle I cannot imagine, were more addicted to prophecies and 
astrological conjurations, dreams, and old wives' tales, than ever they 



DANGERS FROM COMETS. 209 

We may thus find a reference to comets in other places 
where angels are mentioned. When the Psalmist says, ' He 
maketh his angels spirits, and his ministers a flaming fire,' 
he may perhaps have had in his thoughts those mysterious 
celestial visitants, which came he knew not whence, and 
went he knew not whither. Certain it is that a people like the 
Jews would not have been likely to overlook the strangest 
and most impressive of all the objects visible in the heavens. 
Nor is it at all likely that among so many historical narra- 
tives as we find in the Old Testament there would be no 
reference to some of those brilliant comets which were, we 
know, regarded by contemporary nations as strange and 
terrible portents. On the other hand, if the Jew regarded 
comets as angels and ministers of God's wrath, we can very 
well understand that he would speak of them always as 
with bated breath and by names implying their sacred and 
terrible office. Such at least would be the way with a Jew 
of religious tendencies. Others would regard comets with 
indifference. Indeed, Josephus remarks of his fellow- 
countrymen that they were not easily impressed by signs 
from heaven. 'When they were at any time premonished,' 
he says, ' from the lips of truth itself, by prodigies and other 
premonitory signs, of their approaching ruin, they had neither 
eyes nor ears nor understanding to make a right use of 
them, but passed them over without heeding or so much 
as thinking of them ; as, for example, what shall we say of 
the comet in the form of a sword that hung over Jerusalem 
for a whole year ? ' 

Of the feeling with which other nations regarded comets 
it is hardly necessary to speak, so strongly were they 

were before or since. Whether this unhappy temper was originally 
raised by the follies of some people who got money by it, that is to say, 
by printing predictions and prognostications, I know not ; but certain 
it is, books frightened them terribly, such as Lilly's Almanac, Gadhtry's 
Astrological Predictions, Poor Robin's Almanac, and the like ; also 
several pretended religious books, one entitled, ' Come out of her, my 
People, lest ye be partaker of her Plagues ; ' another called ' Fair 
Warning, ' another ' Britain's Remembrancer, ' and many such. ' 

P 



210 MYSTERIES OF TIME AND SPACE. 

possessed with the belief that these objects portended 
trouble to mankind. But, as I have said, it was natural 
that they should think thus, nay, it was impossible that they 
could believe otherwise, so long as they held that the 
heavenly bodies are for signs to men. Even Seneca, who 
was so far in advance of the philosophers of his day as to 
maintain that comets like planets travel in fixed orbits, 
considered that comets were naturally regarded as tokens 
of divine wrath. ' The host of heavenly constellations,' he 
said, ' beneath the vault of heaven, whose beauty they adorn, 
attract no attention; but if any unusual appearance be 
noticed among them, at once all eyes are turned heaven- 
wards. The sun is only looked on with interest when he 
is undergoing eclipse. Men observe the moon only under 
the like condition. The like is true of comets. When one of 
these fiery bodies of unusual form appears, everyone is 
eager to know what it means ; men forget other objects to 
inquire about the new arrival ; they know not whether to 
wonder or to tremble ; for many spread fear on all sides, 
drawing from the phenomenon most grave prognostics.' 

It would be well if our own times were free from these 
idle fears, for it would imply that men were freer from the 
debasing effects of ignorance and superstition. But I do 
not propose to consider here the unwisdom of the belief 
that bodies travelling uniformly in definite paths under the 
influence of the law of gravity should be regarded as 
special ministers warning men either of evil or of good 
approaching them. A man who could believe that Halley's 
comet, whose return was predicted within four weeks in 
1759, and within a few hours (so greatly had the know- 
ledge of the planets and of their attracting powers increased) 
in 1835, was a messenger specially sent from heaven on 
these occasions (or, by parity of reasoning, in its earlier 
visits to our neighbourhood), would believe anything ; 
reasoning would be thrown away on such a one. But there 
is a belief, erroneous no doubt, but not altogether unreason- 
able, which merits such attention as is implied by refutation. 



DANGERS FROM COMETS. 21 1 

I refer to the belief that comets during their approach to 
the earth's neighbourhood or to the sun's may modify terres- 
trial weather either directly or by their action on the sun. 
To this belief, which, by some is regarded as worthy to be 
called a theory, I now propose to apply some of the tests 
which science employs for the purpose of ascertaining the 
truth or falsity of an hypothesis. 

And first it is to be noticed that this theory as originally 
maintained was based on the old Aristotelian doctrine re- 
specting comets, that they are generated in the upper 
regions of the air from a hot and dry exhalation, and so 
consumed. In a book which attracted great attention in the 
earlier part of the present century, Forster's Illustrations of 
the Atmospherical Origin of Epidemic Diseases, the author 
maintains that every unhealthy year since the Christian era 
has been marked by the appearance of a great comet, and 
that no great comet has ever appeared in a healthy year ; 
manifestly believing, with the ancients, that comets act 
malefically by their direct influence on the air. 

So soon as it was shown that the paths of comets do not 
carry them within millions of miles of the earth, or even of 
the outermost fringe of the earth's atmosphere, this faith in 
direct cometary action became untenable. Yet many still 
maintained the theory that a comet acts directly upon the 
earth, because they supposed that the malefic influence of 
comets had been thoroughly established by observation, 
although the manner in which this influence is exerted had 
been misunderstood. 

There was indeed one occasion when apparently men 
had some reason for their fears. It is somewhat amusing, 
now that the fate of Biela's comet has been tolerably well 
ascertained, to think of the terror which that comet excited 
in 1832. Littrow, Professor of Astronomy at Vienna, was 
at the .pains to publish a treatise explaining that these fears 
were unfounded. It had been announced that on October 
29, 1832, the comet would only be about twenty thousand 
miles from the earth's path ; and it was stated that if the 

r 2 



212 MYSTERIES OF TIME AND SPACE. 

earth were within twenty or thirty thousand miles of the 
comet's centre, ' such effects might be felt from the comet 
or from the enormous mass of vapour composing it (com- 
puted to be more than one hundred and fifty times greater 
than the mass of the earth) l as to destroy all animal and 
vegetable life.' But there would have been in reality 
nothing very alarming in the statement unless it had been 
also stated that the earth would be at the point of her orbit 
thus nearly approached by the comet, at the same time that 
the comet was passing. And as a matter of fact astronomers 
knew that the earth would not pass the point of nearest 
approach till November 30, no less than thirty-two days 
after the comet had gone by there. On October 29, the 
earth was about fifty millions of miles distant from the place 
where the two orbits are nearest to each other. 

As to the danger of approach on other occasions Littrow 
wrote as follows at that time (and even his cautious utter- 
ances read strangely in the light of what is now known 
about the comet) : — 'We have already stated,' he said, 'that 
Biela's comet can only come near the earth when it is at its 
least distance from the sun, in the latter part of December. 
But since this proximity of the comet to the sun may just 
as well happen on every other day of the year as in December, 
and since its period is six years two hundred and seventy 
days, or about two thousand five hundred days, after a lapse 
of two thousand five hundred years a near approach (not an 
actual collision) to the comet is probable. 1 say merely pro- 
bable, from which it must not be concluded that such an 
event actually will take place in two thousand five hundred 
years. This result merely means that a man might bet two 
thousand five hundred to ten or to one hundred that the 
comet will not come near the earth for the next ten or one 

1 This is quoted from a periodical of the day, viz., the Penny 
Magazine for October 20, 1832. It is hardly necessary to say that the 
mass of the comet did not approach this amount. Nor did the astro* 
nomers of 1832 make any such mistake as might be inferred from the 
passage quoted. 



DANGERS FROM COMETS. 213 

hundred years. At the end of two thousand five hundred 
years there will be an equal chance that the comet will 
make this next approach, or that it will not. And after two 
thousand five hundred years the chance of its approaching 
the earth will go on increasing, but at so slow a rate that 
many thousands of years must elapse before the comet can 
be really expected.' l 

Since that time Biela's comet has made seven revolutions, 
and although it has not come near the earth (so far at least 
as its head is concerned), yet the comet has undergone 
dissolution, how produced is not known, but probably by 
solar action. In 1872 the earth passed through the comet's 
train of meteoric attendants, but some twelve weeks after 
the comet itself had passed the place where the earth thus 
traversed the family of bodies following along the comet's 
orbit. There was a beautiful display of falling stars, but 
the earth passed on wholly uninjured. 

This was not, as it has been described, a passage of the 
earth through the tail of a comet ; for the meteoric train 
and the tail are entirely distinct appendages, occupying very 
different regions in space. It is worthy of notice, however, 
that the earth has passed through the tail of the comet also, 
without serious consequences. This happened in the case 
of the famous comet of 1861, one of the most magnificent 
ever seen, though the nature of its path was such that the 
comet was not observed by many except astronomers. 
During the night, when, according to the calculations of 
Mr. Hind, superintendent of the Nautical Almanac, the 

1 One does not quite see the force of this reasoning ; or, rather, 
why a mathematician of Von Littrow's strength should content himself 
with anything so vague. What he means is probably this : Roughly 
the earth's period and the comet's contain respectively 365 and 2,500 
days, so that a period of 365 times 2,500 days contains each period a 
certain number of times exactly, viz., 2,500 earth-periods and 2,500 
comet-periods ; hence at the end of this long period the two bodies 
will have returned pretty nearly to the position they had had at the 
beginning, and all possible variations in the manner of the two bodies' 
mutual approach will — speaking roughly — have been gone through. 



214 MYSTERIES OF TIME AND SPACE. 

earth was passing through the tail of this comet, but at a 
great distance from the head, it was noticed by some 
observers that the sky was full of what was described as a 
phosphorescent light. Whether this observation was trust- 
worthy or not, it is certain that if the phenomenon had any 
real existence it was by no means striking. It is equally 
certain that no other effect was observed, and that the earth 
experienced no manner of mischief during its passage 
through that great comet's tail. 

So far as we can judge there is no danger whatever for 
the earth from the passage through a comet's train of meteoric 
attendants, or through the tail. Whether the passage of the 
earth directly through a comet's head would cause any mis- 
chief is as yet doubtful. From what we know of cometic 
structure, however, it seems unlikely that any serious harm 
could happen to the earth, even if she came into direct con- 
flict with the nucleus of the largest comet. Assuming that 
the nucleus of a large comet consists partly of vapour, but 
in the main of meteoric masses such as form the train, only 
more closely set, there might be a downfall of large aerolites 
during the encounter ; and if tens of thousands fell, as in 
the November star-shower tens of thousands of smaller 
bodies fall, it might well happen that here and there a life 
would be lost. But the earth has a large surface. She 
exposes a hundred million square miles to a flight of bodies 
reaching her in any given direction ; so that even though a 
hundred million meteoric masses struck her, that would be 
but one per square mile. The chances against any meteoric 
mass striking a human being would be enormous, even if a 
meteoric shower contained many hundreds of millions of 
masses large enough to penetrate through the atmospheric 
armour of the earth. 

Taking next the question whether a comet may in some 
other way influence the earth, as by its light, or heat, or 
some other emanation, science simply asks another question 
in reply, viz., how can such influence be produced? We 
can measure the light which comes from a comet, even the 



DANGERS FROM COMETS. 215 

brightest, and we find that it is exceedingly small by com- 
parison with the light we get from the full moon. We can- 
not measure a comet's heat, simply because no instrument 
hitherto devised is delicate enough even to afford any in- 
dication of heat from a comet. As for other forms of 
emanation, science knows of none which can come from a 
comet more than from the planets or from the moon, which 
are certainly not regarded as sources of deleterious emana- 
tions. In point of fact, science not only has no a priori 
reasons for supposing that a comet could produce any 
recognisable effects on the earth by its light, heat, or other 
qualities, but has every reason of that kind for believing 
that a comet is absolutely powerless to produce any effect, 
good, bad, or indifferent, on the earth or other planets. 

Of course, it might well be that a posteriori reasons 
might exist for regarding comets as mischievous or dan- 
gerous. If, for instance, it had been found that the ap- 
pearance of a comet was always or generally followed by 
certain effects, as by excessive heat, plague, or pestilence, 
or the like, we should hardly be able perhaps to regard the 
coincidence as accidental. In that case, however unlikely 
it might appear antecedently to the student of science that 
comets could mischievously affect the earth, he would be 
bound to inquire further, in order to see whether the con- 
nection apparently existing between comets and bad years 
of such and such kinds were real or not. It would require, 
let it be at once admitted, a great weight of evidence to 
force anyone really acquainted with what has been dis- 
covered respecting comets to believe that any such con- 
nection exists. This is commonly misunderstood. Many 
think that students of science have come to a foregone 
conclusion in the matter, as in the corresponding case of 
supposed planetary influences. In reality it is simply be- 
cause the student of science recognises the enormous ante- 
cedent improbability of the popular ideas about cometary 
effects upon the earth, that he pays very little attention to 
the evidence which many persons think they find in favour 



2l6 



MASTERIES OF TIME AND SPACE. 



of these ideas. He knows, also, better than those who have 
not studied the subject, what an enormous mass of facts has 
been gathered together, from among which, by due selection, 
what would seem like overwhelming evidence could be found 
in favour of almost any theory. It could be proved to the 
perfect satisfaction of all, except those who have studied 
the subject, that comets produce heat or cold, health or pes- 
tilence, wars and famines, or periods of peace and plenty. 
When we take the entire evidence, we find, as we might 
expect, that it is fairly balanced for all these contradictory 
influences, or, in other words, that there is no evidence at 
all in favour of cometary effects on weather, or on health, 
or on the relations of men and nations amongst each other. 
This is, of course, no new discovery. Ever since modern 
science began — by which I mean science depending on sys- 
tematic observation — it has been known that the idea of 
cometary influences has had no support in observed facts. 
Not to go so far back, the questions which have been asked 
during the past few months were asked half a century ago, 
and then received the same reply which science gives to 
them now. Thus, Von Littrow, writing in 1831 about the 
belief that comets make our seasons warmer, said : ' In 
reply to this assertion I give the years, from 1632 to 1785, 
which were remarkable for the unusual temperature either 
of their winter or their summer, and were likewise distin- 
guished by the appearance of comets : — 



Comet years Temperature 


Comet year; 


Temperature 


1632 


Hot summer 


I702 


Hot summer 


1665 


Severe winter 


1702 


Warm winter \ 


1680 


Severe winter 


1706 


Severe winter 


1682 


Warm winter 


1718 


Hot summer 


1683 


Cold summer 


I7l8 


Severe winter 


1683 


Severe winter 


1723 


Hot summer 


1684 


Cold summer 


1729 


Severe winter 


1689 


Warm winter 


1737 


Hoi summer 


1695 


Cold summer 


1744 


Severe winter 


1699 


Severe winter 


1748 


Hot summer 


1701 


Hot summer 


1764 


Warm winter 



DANGERS FROM COMETS. 



217 



Comet years Temperature 
1766 Severe winter 
1769 Warm winter 
1 77 1 Severe winter 
1774 Hot summer 



Comet years Temperature 
1 78 1 Hot summer 

1783 Warm winter 

1784 Severe winter 
1784 Severe winter 



Here are thirty cases, and it happens that in exactly half 
(the italicised cases) the effect which would be attributed to 
the comet, if the comet had any effect on temperature at 
all, would be an increase of heat, while in the other half 
such effect would be a diminution of heat. It is clear, then, 
so far as the evidence goes, that a comet produces no effect 
one way or the other.' 

Perhaps some reader, noticing that in twenty-two cases 
out of thirty the list shows either a hot summer or a severe 
winter, will suggest that a comet appears in general to cause 
either an excess of warmth in summer or of cold in winter. 
To this the reply simply is that cool summers and warm 
winters are not such noteworthy phenomena as hot summers 
and severe winters, and hence more of the two latter would 
of course be noticed and tabulated than of the two former. 
Indeed, if it would require a great weight of evidence to 
satisfy a student of cometic science that comets had any 
effect at all on temperature, it would require much stronger 
evidence (indeed, evidence quite overwhelming) to satisfy 
him that comets could produce opposite effects, making 
summer hotter and winter colder. 

But though such evidence as the above was given half 
a century ago, and was old even then, we still find the 
question mooted as almost a new one, whether comets affect 
the weather. We had some exceptionally warm weather last 
July, and because a comet was visible, the blame was thrown 
on that celestial visitant. Another comet came, and during 
its visibility the weather was exceptionally cold, yet few 
seem to think that this evidence in one direction should be 
regarded as negativing the supposed evidence in the con- 
trary direction ; while some threw out the startling (and it 
need hardly be said utterly unscientific) notion, that one 



218 MYSTERIES OF TIME AND SPACE. 

comet caused an increase of heat, being of the warm sort, 
while the other, being a cold one, caused the temperature 
to fall. It still remains to be seen what effect the comet 
reported (as I write) from America will produce on the 
weather. 

Are we then to conclude that comets bring with them 
no changes, to our earth or other members of the solar 
system? It appears to me we cannot altogether infer this, 
though the only form of danger which seems to exist is 
fortunately not very marked. 

Though comets can neither injure the earth by falling 
on her surface or by the conflict of their trains or tails with 
her globe, nor by the action of their light, heat, or other 
such influence upon her inhabitants, they might do mischief, 
possibly, by their indirect action. It was long since pointed 
out by Newton that if a comet were to fall directly upon the 
sun, his heat might be so increased after the comet's down- 
fall as to destroy every trace of life on the surface of the 
earth. In Newton's day the cause of the solar heat was not 
well understood. The sun was regarded as a gigantic fire \ 
and the only way in which Newton, or any of his contem- 
poraries, could imagine that a comet could increase the 
sun's heat was by bringing fuel to this monstrous fire. We 
know now that if any great quantity of combustible matter 
could simply be placed upon the sun's surface, his heat 
would be for a while diminished rather than increased, as 
it would be in part occupied in raising the newly arrived 
matter to the sun's own temperature. 

But in another way than Newton had in his thoughts, a 
comet reaching the sun from outer space would cause an 
increase of solar heat ; not as fuel feeding the solar fires, 
but as moving matter adding to the sun's activity by virtue 
of its motion. A comet, if of sufficient mass, might so far 
increase the solar heat as to do mischief to the earth and 
other planets, even though the actual accession of energy 
might be very small indeed compared with the sun's normal 
activity. 



DANGERS FROM COMETS. 219 

Rightly to apprehend the nature of this special danger, 
the reader should compare the statement that a comet falling 
on the sun might do mischief with my former statement that 
a comet falling on the earth would probably do no mischief 
at all, or very little. It might seem at a first view that the 
direct mischief which a comet might cause by falling directly 
on the earth must be far greater than the indirect mischief 
which it could cause the earth by falling on the sun. The 
reason why this is not so is that the body fallen upon has a 
part in the mischief-causing work — indeed, in one case pro- 
duces the whole effect from which mischief may follow. A 
body forming part of a comet (head, tail, or train) which fell 
on the earth would be moving with a certain velocity when 
first its course brought it near enough to the earth to have 
its motion measurably affected by the earth's attraction. 
During the remainder of its course its velocity would be 
increased by the last-named influence, and when finally it 
struck the earth (supposing it able to break its way through 
the resistance of the atmosphere) a portion of its striking 
velocity would be earth-born. But in the majority of cases 
this portion would be small relatively as well as absolutely, 
and in every case it would be absolutely small. The greatest 
possible effect the earth could produce on a body reaching 
her from without would be that which she could produce if 
she were the only orb in the universe, and the body started 
from rest towards her, moving from a very great distance. 
Then she would give to the body a velocity of seven miles 
per second ; that is, the body would strike her surface with 
that velocity. The velocity seems enormous, and is indeed 
some thirty times greater than the velocity of a cannon-ball. 
But even though thousands, or hundreds of thousands, or 
millions of such bodies as form the meteoric train or nucleus 
of a comet reached the earth with this velocity, the total 
effect on the earth would be insignificant (to say nothing of 
the protective effect of the atmosphere). As a matter of 
fact, the earth, not being the only orb in the universe, never 
can give this velocity, or a velocity nearly so great, to a body 



220 MYSTERIES OE TIME AND SPACE. 

approaching her from without. Every such body is, and has 
been for a long time before reaching her, under the much 
greater attractive influence of the sun, and by far the greater 
part of the velocity which any such body has is sun-born. 
Yet even with the velocities generated by the sun at the 
earttts distance, bodies following in the train of a comet, or 
forming part of a comet's head or nucleus, could do little 
harm to the earth. It is because bodies falling on the sun 
are acted on by him much more effectively, that they might 
do harm, more harm indirectly than bodies falling on the 
earth itself could do directly. They cannot reach him with- 
out having been acted on by him over those parts of the 
planetary system which lie within the earth's orbit or nearer 
to him than the earth, nearer than Venus, nearer than 
Mercury, nearer than any planets (if such there are) which 
travel between him and Mercury. Not only may they be 
acted upon up to his very surface as we see it, but it may 
very well be, nay, it almost certainly is the case, that his 
real surface lies far below that apparent surface ; and if this 
is so, a body reaching his actual surface is exposed to the 
yet mightier influence which his giant orb must exert within 
that surface below which no telescope penetrates. Even at 
that surface a body reaching the sun from far remote space, 
under his own attractive influence only, would travel at the 
rate of 360 miles per second. The heat generated when 
a body moving at this rate was brought to rest would be 
enormous, even though the body itself were of small mass. 
When we remember the enormous size of the sun, that the 
surface turned at any instant towards a flight of bodies 
approaching from without is about 2,350,000,000,000 square 
miles, we see that if a comet's nucleus were of the larger 
sort, and contained many millions of millions of rocky 
masses much larger than those which astronomy recognises 
as probably forming the nucleus of Tempel's cbmet (the 
November meteor), the capture within a short time-interval 
of all those masses could not fail to result in a tremendous 
temporary accession of heat by the solar mass. For a short 



DANGERS FROM COMETS. 221 

time — it might be for a few days only, or for a few hours 
even — the emission of solar heat would bejjreatly increased. 
Without any very inordinate conceptions as to the total 
mass of the destroyed comet, we can see that the solar heat 
might for a day or two be doubled or even increased in 
much greater degree. He would return presently to his 
usual condition, but in the meantime the earth's inhabitants 
would have suffered greatly, even if they had not been (as 
they well might be) destroyed altogether by excess of heat. 

But, it may be said, the dangers here described are 
wholly imaginary. No comet of the larger sort ever has 
fallen, or ever can fall, on the sun. We know that thou- 
sands of comets have appeared in our skies without any 
such ill effects. We know also that our sun is one of many 
thousands of suns, all of which we must assume are equally 
exposed to the dangers described ; yet all shine steadfastly 
in the heavens. Neither the comets which science has 
observed and studied, nor the stars whose lustre has been 
determined and watched, tell us anything to confirm the 
dismal anticipations suggested by the above considera- 
tions. 

It so happens, however, that comets and stars have 
agreed in showing that the danger exists, though they agree 
in indicating that it is small and remote. Or rather, the 
evidence given by the stars, if it really bears on the danger 
we are considering, shows that the chance of mischief is 
small, but that should the mischief occur it would be very 
great, if not absolutely destructive. 

First as to the evidence given by comets. 

Most comets travel on paths which nowhere approach 
within many millions of miles of the solar orb. Trie effects 
mentioned this year as likely to have been produced by 
cometic action on the sun could never have been imagined 
by any except those utterly ignorant of matters astro- 
nomical j where persons not so ignorant suggested dangers, 
it was with the intention of acting upon public credulity in 
such matters, Every astronomer knows that not one of the 



222 MYSTERIES OP TIME AND SPACE. 

comets of the year 1881 could have produced the slightest 
measurable effect upon the sun. 

But there have been comets which have approached so 
near to the sun's surface as to suggest unmistakably the 
possibility that a comet may one day be absorbed by the 
sun. Such was the comet of 1668, which, according to the 
rough observations of Goa, in India, passed within 40,000 
or 50,000 miles of the sun's surface. The comet of 1843 
passed within 190,000 miles of the sun's surface according 
to some estimates, but according to others went nearer. 
When we consider that these estimates refer to the centre 
of the comet's head, and that a comet is not a point but a 
very large object, while we know that outside the visible 
surface of the sun the prominence region extends many 
thousands of miles, we see that such comets as the above- 
named may be regarded as having to all intents and pur- 
poses absolutely grazed the surface of the sun. 

But this is far from being all. In February 1880 a comet 
appeared whose path was very similar to that pursued by 
the comet of 1843. Mr. Hind, the superintendent of the 
Nautical Almanac, examining the observations made by 
Dr. Gould at Cordoba, and by Mr. Ellery at Melbourne, 
as well as the places noted by Mr. Gill, of Cape Town 
Observatory, obtained in each case for the comet of 
February 1880 a path sensibly the same as that of the 
comet of 1843. Professor Weiss, of Vienna, was led to a 
similar conclusion ; while we learn that Professor Winnecke, 
judging from a comparison of the orbit of the great comet 
of 1843 with Gould's position on February 4, and Gill's 
later rough ones, is of opinion that the identity of the comets 
of 1843 and 1880 hardly admits of a doubt 

Now the comet of 1843 was not expected to return so 
soon as 1880. Professor Hubbard, of Washington, assigned 
to it a period of revolution of 533 years. He showed, in- 
deed, that a period of 200, or 175, or even 150 years, might 
be reconciled with the observations ; and Dr. Gould has 
shown that the period of thirty-seven years, which would 



DANGERS FROM COMETS. 223 

correspond with the return of the comet in 1880, involves 
no very important correction of any single observation made 
on the comet of 1843. Still there is this great difference 
between the interpretation of the comet's observed motions 
with the longer and the shorter periods. Where the longer 
periods are used the discrepancies are pretty equally dis- 
tributed in different directions — one observation sets the 
comet slightly in advance of the position calculated from 
the assumed period, another sets the comet slightly be- 
hind its calculated place ; one sets it slightly on one side, 
another slightly on the other side of its computed orbit. 
But when one of the shorter periods is employed this is no 
longer the case. The discrepancies, though slight, are all 
in one direction. Every astronomer recognises the import- 
ance of this difference. 

Assuming then that one of the longer periods, say a 
period certainly exceeding 100 years, must most probably 
be assigned to the comet of 1843, while yet we cannot reject 
the evidence showing the identity of the comets of 1843 and 
1880, we are led to the conclusion that from some cause or 
another the period of the comet has undergone a remark- 
able diminution. We can hardly imagine that there are 
two different comets travelling in the same track. It is 
true we find meteoric flights travelling in the same tracks 
after a comet, but we have nothing which seems to render 
it likely, or indeed conceivable, that two comets would be 
associated in this way. We seem forced to accept as at any 
rate far more probable the conclusion that the comets of 
1843 and 1880 are really one and the same object, but that 
the period, formerly much larger, has been reduced to 
thirty- seven years. 

But there is only one way in which a comet's period can 
be reduced so greatly, viz., by a cause diminishing the 
comet's velocity at some point of its orbit. Moreover, the 
place where the velocity is thus affected must lie in or near 
that part of the comet's orbit which remains almost un- 
changed. The track pursued by the comet of 1880 during 



224 MYSTERIES OF TIME AND SPACE. 

its visibility was almost precisely the same as that pursued 
by the comet of 1843. Hence the comet of 1843 must 
have been disturbed somewhere along that part of its track 
which thirty-seven years later was traversed by the comet of 
1880. In the very midst of this part of the track lies the 
point where either track approaches nearest to the sun — 
the perihelion of the orbit as it is technically called. Some- 
where near this point, most probably at this very point, 
the velocity of the comet of 1843 must have been reduced. 
Now we have seen that at this part of its path the comet was 
very close indeed to the sun, so close that even the centre 
of the head must have passed through the surface of the sun. 
We can understand then that the comet may here have been 
retarded by the resistance of the matter forming the solar 
appendages (the prominences and the corona), even if not 
still more effectively retarded by resistance experienced at 
the actual surface of the sun. If so retarded in 1843 the 
comet must have been still further retarded in 1880, and its 
period still further reduced. If so, it will probably return 
before the end of the present century, then again after a shorter 
interval, and so after gradually shortening intervals until be- 
fore very long the comet will be finally absorbed by the sun. 
Now all this implies no great danger either for the sun 
or the earth. If we assume that our conclusion is ab- 
solutely correct, and that the comet will before long — say in 
less than a century — be absorbed by the sun, still there are 
abundant reasons for believing that the mischief which could 
possibly accrue to the earth can be but small. The comet, 
according to our assumption, was effectively retarded in 
1843. At that time no inconsiderable portion of its motion 
must have been transformed into solar heat Yet we know 
that there was no such accession of solar heat as could be 
felt by all, none even that science could measure. Nor was 
there any such accession of solar heat in 1880, when the 
comet must have been still further retarded. There is then 
every reason to believe that whatever danger some comets 
may bring to the solar system, the comet of 1843 i s not one 



DANGERS FROM COMETS, 225 

of the very dangerous ones. Its course brings it menacingly 
near to the solar orb, but its mass and constitution appear 
to be such that its final absorption by the sun will not 
involve any serious danger to the solar system by increase 
of the sun's heat. 

When we consider, however, how vastly the comet of 
1843 has been exceeded in volume and presumably in mass 
by other known comets, and the wide range of disparity in 
splendour among comets already observed (showing that 
probably even the largest observed may be but small com- 
pared with some comets which exist but have not yet been 
seen), we see that the kind of danger shown by the motions 
of the comet of 1843 to be real enough, may in the case of 
other and much larger comets be not only real but great. 
Such a comet, for instance, as that of 181 1, which, though 
it never approached the sun within 90,000,000 miles, yet 
displayed greater splendour and greater cometic develop- 
ment than comets which have all but grazed the solar sur- 
face, would be a very dangerous visitor if its course chanced 
to be so directed as to carry it straight towards the sun. 
And there may well be comets as far exceeding that of 
181 1 as this exceeded the comet of 1843, while the course 
of any comet may well chance to be so directed as to carry it 
straight towards the very centre of the sun instead of passing 
grazingly by his orb as did the comet of 1843. Of course 
the chance of a very large comet visiting the solar system 
on just such a course is exceedingly minute. Still the event 
is altogether possible. There can scarcely be a doubt that 
if the event occurred the result would be disastrous for the 
present inhabitants of the solar system. The downfall of 
millions of millions of masses, each weighing many tons (a 
fair supposition as to the average weight of the meteoric 
attendants on so large a comet as we are considering), at the 
rate of 350 or 360 miles per second, upon the sun's orb, 
could not fail to be an enormous, though short-lasting, 
accession of solar splendour and of solar heat, a change 

Q 



226 MYSTERIES OF TIME AND SPACE, 

which could not but prove destructive to every form of life 
existing on the earth or any other inhabited planet. 

The chance of such a catastrophe is small It is so 
small that not one sun in millions might be expected to suffer 
in this way during thousands of years. (For we must remember 
that our sun is one of a very large family of suns, and that 
whatever danger he is exposed to, threatens presumably 
each member of that family.) May we not in this way test 
at once the reality and the extent of the danger ? If any 
sun among the millions, the tens, nay, the hundreds of 
millions, 1 visible in the telescope should sustain the .direct 

1 It is commonly stated that within the range of the gauging 
telescopes of the Herschels as many as twenty million suns are visible. 
This estimate, due to the French astronomer Chacornac, falls far short 
of the truth. Argelander was able, with a telescope less than three 
inches in diameter, to chart more than 300,000 stars in the northern 
skies. From observations of my own I am satisfied that if the survey 
with that instrument had been carried on only upon the darkest and 
clearest nights at least 500,000 stars would have been seen in the 
northern hemisphere, or a million stars in the entire heavens, or more 
than 150 times as many as are visible to the naked eye. Now, at a 
most moderate computation, the Herschelian eighteen-inch gauging 
telescopes have twenty-five times the light-gathering power of the puny 
instrument used by Argelander. A star which would be just visible 
with the eighteen-inch telescope would be five times as far away as one 
which would be just visible with the 2\ -inch one. The stellar domain 
ranged over by the larger telescope would therefore be 5 times 5 times 
5 times or 125 times as large as that surveyed by the smaller. Apart 
then from any extinction of light in its passage through space, and 
assuming an equal distribution of stars within the range of the larger 
telescope, 125 times more stars would be shown by the larger than by 
the smaller instrument. Now, allowing the fullest weight to the elder 
Struve's theory of extinction, or rather to the evidence on which it is 
based (which will equally well be explained by a diminishing richness 
of star-distribution at great distance), we yet cannot suppose that the 
total number of stars within range of the great gauging telescopes 
would be reduced from 125 to barely 20 millions. Probably there are 
at least a hundred millions of stars within the range of those telescopes, 
and a thousand millions within the range of the great telescope of Lord 
Rpsse, 



DANGERS FROM COMETS. 227 

impact of a very large comet, and should thereby for a 
short time increase greatly in heat and lustre, that sun would 
during that time be visible without telescopic aid. Pro- 
bably even the faintest star which the most powerful telescope 
can just show us, would become visible to the naked eye 
during such an outburst of light and heat. 

Turning to the stars to see what evidence they have 
given, we find that there have been occasionally just such 
changes among the stars as we should be led to expect from 
what the comets have taught us. We find that, on the one 
hand, some stars have suddenly increased in lustre so greatly 
as to pass from absolute invisibility to great brightness (in 
one or two cases even to a brightness exceeding that of a first 
magnitude star) ; while on the other hand these cases have 
been so few when the enormous number of stars is taken 
into account, as to show that the danger in the case of any 
given star is exceedingly small. Among all the hundreds of 
millions of suns working steadily at their task of ruling and 
nourishing the worlds that circle around them, not one in a 
million has during the last three thousand years met with an 
accident of the kind considered, even if we assume that 
every appearance of a so-called ' new star ' is to be regarded 
as in reality a case of solar outburst, and has been in reality 
brought about by cometic or meteoric downfall. Consider- 
ing that of two such cases submitted to spectroscopic inves- 
tigation (the so-called new star seen in Cygnus in November 
1876) one proved to be no new star at all, while in the other 
(the new star seen in Corona in May 1869), though it was 
undoubtedly a case in which a sun blazed for a time with 
several hundred times its normal splendour, the change may 
possibly have resulted from some other cause of danger to 
which our sun may not be exposed, we see that, so far as 
probabilities are concerned, the danger that the solar system 
may be ruined by a solar outburst of some sort is exceed- 
ingly small. The only kind of danger to which, so far as 
we can judge, our sun is exposed, that from cometic down- 
fall upon his globe, has not yet been proved to be serious 

9 2 



228 MYSTERIES OF TIME AND SPACE. 

in itself; while assuming that such a cause might produce a 
great increase of solar light and heat for a while, we learn 
from the stars that the actual cases of such change among 
all the stars from all causes are very few in number, con- 
sidering the enormous number of the stars. The chances 
are certainly not one in a million that our sun will undergo 
any change of the kind considered during the next ten 
thousand years, even if the sun be supposed to be ante- 
cedently as much exposed to such change as those other 
suns which appear to have undergone it. But the constancy 
of the solar light and heat during the past five thousand 
years, and even (judging from the geologic record) during 
hundreds of thousands of years, affords in reality strong 
evidence that he is less exposed than some at any rate 
among the suns to dangers of this kind. Indeed, it is worthy 
of notice that almost all the so-called new stars — that is, if 
our views are correct, almost all the suns that have under- 
gone a change destructive to life on their dependent worlds — 
occupy a certain definite region of the heavens lying near 
the edge of the Milky Way. Taking this into account, it 
may be said, in fine, that the danger of our earth's destruc- 
tion by fire, the elements dissolving under the fervent heat 
of the comet-struck sun, is so small that it may to all intents 
be valued at * almost naked nothing.' 



229 



THE WORLD'S END. 

Great talk among people how some of the Fanatiques do say that 
the end of the world is at hand, and that next Tuesday (Dec. 2, 1662) 
is to be the day. — Pepys Diary. 

In the year 1000 a.d. it was almost the universal opinion 
that the world approached its end. Early Mother Shiptons 
had indicated that as the fateful year. Satan had been 
chained for a thousand years, and was to be loosened when 
the thousand years were complete. The end of the world 
was to be brought about by him indirectly, for his temporary 
triumph was to lead to the second coming of Christ, the 
Day of Judgment, and the end of all things terrestrial. The 
anticipation of these events caused natural phenomena, such 
as are occurring all the time, to assume a more than usually 
portentous aspect. Just as last year, when, according to the 
Shipton prophecy, our world was to come to an end, every 
one who believed in the prophecy found in the weather re- 
ports from different parts of the earth proof positive, or at 
least confirmation strong, of the threatened end — men's 
hearts failing them for fear because of earthquakes, storms, 
and so forth, which ordinarily pass without attracting special 
attention ; so in the year 1000 every meteorological and 
celestial phenomenon was anxiously watched as the possible 
precursor of the coming catastrophe. A comet appeared and 
was visible for nine days, and everyone began to ask (like 
Fanny Squeers), ' Is this the end ?' A wonderful meteor was 
seen, and men's frightened fancies enabled them to see what 



230 MYSTERIES OF TIME AND SPACE. 

men of science seldom have the opportunity of observing 
now during meteoric displays. ' The heavens opened,' we are 
told, ' and a kind of flaming torch fell upon the earth, leaving 
behind a long track of light like the path of a flash of light- 
ning. Its brightness was so great that it frightened not only 
those who were in the fields, but even those who were in the 
houses. As this opening in the sky slowly closed, men saw 
with horror the figure of a dragon, whose feet were blue, and 
whose head seemed to grow larger and larger.' A terrible 
picture accompanies this description. There is the meteor 
track, with various coruscations and widenings, so arranged 
as to correspond with the figure of a dragon assigned to the 
portentous object ; but as the resemblance might not seem 
absolutely convincing to unimaginative persons, a dragon 
to match is set beside the celestial apparition, and this crea- 
ture is labelled for the benefit of the inexperienced, ' Serpens 
cum ceruleis pedibus.' 

It is exceedingly probable that if general literature had 
reached as widely then as it does now, the fears entertained 
in the year iooo would have surpassed in intensity those 
which have been engendered since that time by successive 
predictions of the world's approaching end. But the great 
bulk of the population here and elsewhere probably heard 
very little of these terrible forewarnings. They had many 
other things to attend to in those ' good old times,' and some 
of their surroundings might very likely have suggested that 
they could not be much worse off if the world should 
actually perish at that time. As for their betters, they also 
were pretty busily engaged plundering each other and fight- 
ing with such zeal that manifestly for a considerable number 
the end was likely to come at least as soon as the general 
destruction threatened by the prophets. At any rate, though 
we have clear evidence that many believed in the predicted 
end of the world (indeed it was thought very wicked to be 
in doubt about it), matters went on much as usual ; the 
year iooi began, and still the world endured, with every sign 
of continuing. 



THE WORLD'S END. 231 

The belief that the world would come to an end in the 
year 1000 was associated with, if not absolutely derived 
from, a much older belief entertained by the earliest astro- 
nomers of whom any records remain to us. They considered 
that certain cyclic periods of the planetary motions begin 
and end with terrestrial calamities, these calamities being of 
different characters according to the zodiacal relations of the 
planetary conjunctions. Thus the ancient Chaldeans taught 
(according to Diodorus Siculus) that when all the planets 
are conjoined in Capricornus the earth is destroyed by 
flood ; when they are all conjoined in Cancer the earth is 
destroyed by fire. But after each such end comes the be- 
ginning of a new cycle, at which time all things are created 
afresh. A favourite doctrine respecting these cyclic destruc- 
tions was that the period intervening between each was the 
Annus Magnus, or great year, required for the return of the 
then known planets to the position (of conjunction) which 
they were understood to have had at the beginning of the 
great year. According to some this period lasted 360,000 
years ; others assigned to it 300,000 years, while according 
to Orpheus it lasted only 120,000 years. But it was in 
every case a multiple of a thousand years, and the subordi- 
nate catastrophes were supposed to divide the great year 
into sets of so many thousand years. 

In Plato's ' Timseus ■ we have some account of the Egyp- 
tian ideas concerning these successive world-endings, though 
minor catastrophes only are referred to ; but when Solon 
described to the Egyptian priests Deucalion's flood, and 
counted for them the generations which had elapsed since 
it occurred, an aged priest said to him : ' Like the rest of 
mankind the Greek nation has suffered from natural convul- 
sions, which occur from time to time according to the position 
of the heavenly bodies, when parts of the earth are destroyed 
by the two great agents, fire and water. At certain periods 
portions of the human race perish in the waters, and rude 
survivors too often fail to transmit historical evidence of 
the event. You Greeks remember one record only. There 



232 MYSTERIES OF TIME AND SPACE. 

have been many. You do not even know at present any- 
thing of that fairest and noblest race of which you are a seed 
or remnant.' The aged priest then read from Egyptian 
annals the records of events which had happened in Greece 
9,000 years before ; he described the founding of the city 
of Sais 8,000 years before ; and this account, registered in 
their ancient and sacred records, Solon read at leisure. 
The most remarkable of the earth's cataclysms were there 
described, including the destruction by flood of the great 
island of Atlantis. This was described as a continent oppo- 
site the Pillars of Hercules (the Straits of Gibraltar), larger 
in extent than Lybia and Asia together (!), and was on the 
road to other islands, and to a great continent of which the 
whole of the Mediterranean Sea was then but the harbour. 
Within the Pillars the empire of Atlantis reached to Egypt 
and Tyrrhenia. In remote times this mighty power was 
arrayed against Egypt and Hellas, and all those countries 
which bordered on the Mediterranean. Greece bravely re- 
pelled the invaders and freed all nations within the Pillars. 
Some time after there was a great earthquake, and the 
warrior races of Hellas were drowned — the great island of 
Atlantis also disappeared, being submerged beneath the sea. 

The conflagrations and deluges by which portions of the 
earth, and at times the whole earth, were destroyed, were 
believed to be intended for the regeneration of the world. 
After each catastrophe, men were created afresh free from 
vice and misery ; but gradually they fell away from this 
happy state to a condition of immorality, which rendered a 
new decree of destruction necessary. 

Lyell notes that the sect of Stoics adopted most fully the 
system of catastrophes thus designed for the alternate de- 
struction and regeneration of the world. They taught that 
they were of two kinds — 'the cataclysm, or destruction by 
water, which sweeps away the whole human race, and anni- 
hilates all the animal and vegetable productions of nature ; 
and the epyrosis, or destruction by fire, which dissolves the 
globe itself. From the Egyptians also they derived the doc- 



THE WORLD'S EtfD. 233 

trine of the gradual debasement of man from a state of 
innocence. Towards the termination of each era the gods 
could no longer bear the wickedness of men, and a shock of 
the elements, or a deluge, overwhelmed them ; after which 
calamity Astrasa again descended on the earth, to renew the 
golden age.' 

That the partial destructions of the earth, whether by 
flood or fire, were associated with the movements of the 
heavenly bodies is evident from the fact that, wherever we 
meet with these ideas, whether in Egyptian, Assyrian, Indian, 
or Chinese records, direct reference is always made to the 
conjunction of the planets, the position of the sun and moon, 
and occasionally to the apparition of comets and the fall 
of meteoric bodies. The following account of the Chinese 
Flood, attributed to the reign of Yu, is traced in the order of 
Hangshan, a mountain on which for many ages annual sac- 
rifices were made by the ancient emperors of China. ' The 
great and little islets and inhabited places,' says the vene- 
rable emperor of the house of Hia, * even to their summits, 
the abodes of the beasts and birds and all beings, are widely 
inundated. I repose on the top of the mountain Yohlu. 
By prudence and labours I have communicated with spirits. 
I know not the hours, but repose myself only amid incessant 
labours. By the dark influence of sun and moon the moun- 
tains Hwa, Yoh, Tai, and Hang alone remain above the 
waters. Upon them has been the beginning and end of my 
enterprise. When my labours were completed I offered a 
thanksgiving sacrifice at the solstice. My affliction has 
ceased ; the confusion in nature has disappeared ; the deep 
currents coming from the south flow into the sea. The 
flood began at equinox. The skies rained meteoric showers 
of iron of extraordinary duration.' Some portions of the 
country remained under water several years until B.C. 2233, 
when canals ordered to be cut by the Emperor Ta Yu con- 
veyed to the sea the immense bodies of water which had 
been precipitated upon and overflowed so .large a part of 
China. By this means-river beds were finally cut, shedding 



234 MYSTERIES OF TIME AND SPACE. 

water in new directions, and continued to be worn deeper 
by the receding flow, until the whole country was tolerably 
free from inundation. 

Sir Charles Lyell remarks of this flood that it rather 
interrupted the work of agriculture than involved any wide- 
spread destruction of the human race. Mr. Davis, who 
accompanied two British embassies to China, points out that 
'even now a great derangement of the waters of the Yellow 
River might cause the flood of Yaou to be repeated, and lay 
the most fertile and populous plains of China under water.' 
It is noteworthy, however, that in the ancient records the 
action of the sun and moon, presumably in raising tides, is 
mentioned, while meteoric showers are distinctly associated 
with the occurrence of the flood — though whether they came 
at the beginning of the disturbance, or simply occurred 
while the waters were out over the plains of China, does not 
clearly appear. 

After the threatened but not accomplished destruction 
of the world in the year a.d. iooo, comets were for a while 
looked on with suspicion, an idea appearing to prevail that 
the torch which was to light the final conflagration would be 
a cometic one. For several centuries, however, no comet 
came near enough to the earth or sun to excite any serious 
terrors founded on observed astronomical relations. But 
the comet of 1680 really presented characteristics which 
suggested dangers even to men of science. It was a comet 
of remarkable appearance ; its course seemed at first directed 
full upon the sun ; and though in those days it was the 
erroneous idea that the comet might supply an undue 
amount of fuel to the central fire of the solar system, which 
chiefly occupied men's thoughts (even Newton sharing the 
idea), the danger from which the solar system then escaped 
was considered to be real and serious. 

In the year 1773 a report got abroad — how engendered 
is not known — that Lalande, one of the ablest mathema- 
ticians of the day, had predicted the end of the world, as 
the result of a collision to take place between a comet and 



THE WORLD'S END. 235 

the earth. We say it is not known how the report got abroad. 
The circumstance which gave rise to the report, is, however, 
well known, though avowedly there was nothing in it to have 
suggested special anxiety. The difficulty is to connect the 
circumstance with the exaggerated terrors presently excited. 
It had been announced that Lalande would read before the 
Academy of Sciences a paper entitled ' Reflections on those 
comets which can approach the earth.' It would be difficult 
to inquire how the report of this came gradually to be 
changed into the definite news that in the year 1773 — nay, 
the very day was named, on May 20, 1773 — a comet would 
encounter and destroy the earth, did not recent experience 
show how a statement of one kind may be changed — through 
carelessness, not through wilful misrepresentation — into a 
statement of an entirely different kind, when (in its later 
form) it seems to indicate the approach of some great danger 
to the earth. Plantamour, lecturing in 1872 about comets and 
meteors, says that the comet of 1862 passed near the earth's 
orbit j that along its track are travelling millions and millions 
of meteoric bodies ; and that when the earth crosses its 
track meteoric displays may be expected ; adding that the 
next display of the kind may be expected on or about 
August 11 or 12. Presently the news is travelling about 
that on August 12, 1872, a comet will fall upon the earth 
and we shall all be destroyed. Who gave to Plantamour's 
true and innocent statement this false and mischievous 
form ? No one can say ; no one can point out where or 
how the true became merged into the misleading, the mis- 
leading into the incorrect, the incorrect into the utterly false. 
But the terrors excited were none the less real that no one 
could tell whence they came or how they were generated. 

Once such fears have been excited, it seems useless to 
attempt to quiet them, at least among the hopelessly ig- 
norant, who unfortunately are so numerous and so readily 
made the victims of idle terrors. Lalande published in the 
Gazette de France of May 7, 1773, the following advertise- 
ment, to quiet, as he hoped, the public mind : ' M. Lalande 



236 MYSTERIES OE TIME AND SPACE. 

had not time to read his memoir upon comets which may 
approach the earth and cause changes in her motions ; but 
he would observe that it is impossible to assign the epochs 
of such events. The next comet whose return is expected 
is the one which should return in eighteen years ; but it is 
not one of those which can hurt the earth/ But this tole- 
rably explicit statement had no effect. M. Lalande's study 
was crowded day after day with anxious inquirers. A num- 
ber of pious people, of whom a contemporary journal made 
the very rude remark that ' they were as ignorant as they 
were imbecile,' begged the Archbishop of Paris to appoint a 
forty days' prayer to avert the threatened danger, which for 
some reason they agreed was to take the form of a mighty 
deluge. And he would have complied with their request 
only he was told by members of the Academy that he would 
bring ridicule upon himself and upon science if he did so. 

It was at this time that Voltaire wrote his well-known 
' Letter on the pretended Comet.' It ran thus : — 

Grenoble, May 17, 1773. 

Certain Parisians who are not philosophers, and who, if we are to 
believe them, will not have time to become such, have informed me 
that the end of the world approaches, and will occur without fail on 
the 20th of this present month of May. They expect that day a 
comet, which is to take our little globe from behind and reduce it to 
impalpable powder, according to a certain prediction of the Academy 
of Sciences which has not yet been made. Nothing is more likely than 
this event, for James Bernouilli, in his treatise upon the comet of 1680, 
predicted expressly that that famous comet would return with a terrible 
uproar on May 19, 1790 ; he assured us that its peruque indeed would 
signify" nothing mischievous, but that its tail would be an infallible 
sign of the wrath of heaven. If James Bernouilli mistook, it is, after 
all, but a matter of fifty-four years and three days. Now, so small an 
error as this being regarded by all geometricians as of little moment in 
the immensity of ages, it is manifest that nothing can be more reason- 
able than to hope for the end of the world on the 20th of this present 
month of May, 1773, or in some other year. If the thing should not 
come to pass, ' omittance is no quittance ' (ce qui est differe, n' est pas 
perdu). There is certainly no reason for laughing at M. Trissotin, 



THE WORLD'S END. 237 

triple idiot though he is {totit Trissotin qtiHl est), when he says to 
Madame Philaminte (Moliere's Femmes Savantes, act. iv sc. 3) : — 

' Nous l'avons en dormant, madame, echappe belle ; 
Un monde pres de nous a passe tout du long, 
Est chu tout au travers de notre tourbillon ; 
Et s'il eiit en chemin rencontre notre terre, 
Elle eut ete brisee en morceaux comrae verre.' 

' A comet coursing along its parabolic may come full tilt against 
our earth. ' But then, what will happen ? Either that comet will have 
a force equal to that of our earth, or greater, or less. If equal, we 
shall do the comet as much harm as it will do us, action and reaction 
being equal ; if greater, the comet will bear us away with it ; if less, 
we shall bear away the comet. This great event may occur in a 
thousand ways, and no one can affirm that our earth and the other 
planets have not experienced more than one revolution through the 
mischance of encountering a comet on their path. The Parisians will 
not desert their city on the 20th inst. ; they will sing songs, and the 
play of ' The Comet and the World's End ' will be performed at the 
Opera Comique. 

Singularly enough, something even more preposterous 
than what the great wit had thus suggested did actually 
occur on this occasion. The fears inspired by the predicted 
approach of the comet were so great that speculators took 
advantage of the terrors of the ignorant, and absolutely per- 
suaded many that the priesthood had, by special intercession, 
obtained the privilege of dispensing a number of tickets 
for seats in Paradise ; and these pretended tickets were sold 
at a very high rate. It would be interesting to inquire what 
idea was entertained by those who purchased these tickets 
as to the way in which they were to be used, to whom pre- 
sented, at what time, and where. 

The story to which I have just referred was quoted by a 
Parisian professor in 1832, when a similar scare prevailed in 
France. It had been announced that the comet of 1826 
(Biela's) would return in 1832 ; and it had also been stated 
that the path of the comet intersected, or very nearly inter- 
sected, the path of the earth. This was immediately inter- 
preted to signify an approaching collision between the earth 



238 MYSTERIES OF TIME AND SPACE, 

and the comet, though nothing of the kind was implied. 
These fears, said the worthy professor, may produce effects 
as mischievous as those produced by the cometic panic in 
1773, unless the authority of the Academy apply a prompt 
remedy ; and this salutary intervention is at this moment 
implored by many benevolent persons. 

At the present time, the end of the world is threatened 
in more ways than one. The methods of destruction are 
incongruous ; but that is a detail hardly worth considering. 
If Scylla does not destroy us, Charybdis is bound to do 
the work, and vice versa. There is no escape for us. 

A few months ago the prophecy of Mother Shipton was 
chiefly feared. But as the world certainly did not come 
to an end in 1881 (though Gerald Massey says Mother 
Shipton's prophecy— which she never made by the way- 
was really fulfilled) we must now look for the world's de- 
struction in other ways. 

And first we see it clearly indicated in the Great 
Pyramid. By slightly altering the dates accepted by his- 
torians, adding a few years in one place and taking off a few 
years in another, it can be proved to demonstration that 
the number of inches in the descending or entrance pas- 
sages, as far as the place where the ascending begins, is 
equal to the number of years from the descent of man to the 
Exodus ; and that the ascending passage contains as many 
inches as there are years from the Exodus to the beginning 
of the Christian era. (The rest of the descending passage, 
as far as the bottomless pit, or the pit with the ruin- hidden 
bottom — it is the same thing — clearly represents the progress 
of the rest of the human race downwards.) This being so, 
of course it follows that the grand gallery represents the 
Christian era. This gallery has a length of T882 inches, or, 
according to recent statements (not new measurements), 
i88i'59. Hence, in the year 1882, or more exactly at the 
time 1 88 1*5 9, which corresponds to 1881 years + 7 months 
-f- 2 \ days, or to midnight between August 3rd l and 4th, the 
1 Astronomically the second day in August ends at noon August 3, 



THE WORLD'S END. 239 

Christian era is to end. The reader is not to be alarmed, 
however, by this seemingly precise statement. As the time 
has drawn nearer, the pyramidalists have seen fit to add 
fifty years (more or less, according to circumstances) during 
which the end is to be finally brought about ; August 3 will 
only mark ' the beginning of the end.' Still, it may fairly be 
presumed that something significant will happen about that 
time. Possibly some remarkable person, or person who is 
hereafter to be remarkable, will be born at midnight August 
3 ; in which case it seems possible that the world might 
remain in ignorance of the fact for a year or two. 

But next the planets take their turn. The terrible words 
' perihelion conjunctions ' are heard with appalling effect. 
It is true they are entirely without meaning ; science knows 
nothing about perihelion conjunctions ; but that is nothing 
— any name is good enough to conjure by. Let us see 
what perihelion mischief is in store for us. 

Jupiter was in perihelion on September 25, 1880 ! 
'The perihelia of other planets in 1881 occurred' (this is 
not a scientific mode of presenting the matter ; but that is 
not the fault of the prophets — they speak as correctly as 
they can) 'as follows : Mercury, February 21 ; Venus, 
March 6 ; Mercury, May 20 ; Mars, May 26 ; Mercury, 
August 16 ; Venus, October 16 • Mercury, November 12.' 
This was very dreadful ; though somehow the earth escaped 
that time. Imagine Mercury being four times in perihelion 
in one year. We may perhaps find an explanation in the 
circumstance that he completes the circuit of his orbit more 
than four times a year, and must pass his perihelion each 
time ; but science tries to explain everything, and we must 
not be too precise in such matters. The year 1882, in 
which we are more interested, is even worse. Mercury has 
already been in perihelion, viz. on February 8 ; then we 
have March 25 (April 9?), Uranus; May 7, Mercury; 
August 3, Mercury ; October 29, Mercury again ; and 
absolutely on December 6 Venus transits the sun's disc ! 
Something will surely come of this, if we only live to see it. 



24P MYSTERIES OF TIME AND SPACE. 

But worse remains behind. 'In August 1885, Saturn 
will be in perihelion ! ' * Neptune is in apparent perihelion ' 
(whatever that may mean). 'from 1876 to 1886, the height 
(?) being about 1881^ ! ' 'Those skilled in astronomy in- 
form us it is fully 6,000 years since the occurrence of a 
similarly powerful situation, although conjunctions and peri- 
helia have occurred at more frequent intervals of time. To 
form an approximate opinion of what the earth is liable to 
experience at such periods, we must review the records of 
effects attending similar situations, remembering that with 
the ripening of our planet the effects upon the earth and its 
inhabitants will be more generally distributed.' 

This being so, these perihelia occurring in so unusual 
a way, being also rendered very terrible by being called 
perihelion conjunctions, and the dependence of terrestrial 
disturbances on planetary motions being too obvious to be 
worth proving, we have only to consider what has happened 
during past floods, earthquakes, and so forth, to see exactly 
what is in store for us pretty soon. Science, which is 
always too particular in such matters, may perhaps show 
that whatever influences the outer and larger planets may 
produce on the earth (it is very doubtful whether they pro- 
duce any except very slight deviations from her mean track) 
cannot be effectively greater when the planets are in peri- 
helion than when they are in aphelion ; that terrestrial 
disturbances have nothing whatever to do with these 
relations ; and that as perihelion passages and planetary 
conjunctions are occurring every year, earthquakes and 
floods could not possibly occur in years when there were no 
such phenomena : but the prophets have nothing to say to 
all that ; they calmly go on to describe the various terrestrial 
disturbances which have occurred, regarding any attempt to 
show that there is the slightest real connection between the 
planetary movements and the earth's throes as quite un- 
necessary. 

Here, however, is the summing up of the planetary 
prophecies by one of the most earnest, and therefore wildest 



THE WORLD'S END. 24 f 

of the prophets. ' In cases of planetary attraction, the 
earth's crust becomes attracted as a solid whole. Its fluid 
and aerial envelope responds when irregularly attracted, by 
oscillating in high and low tides, alternating with unequal 
pressure. We are approaching both stellar and planetary 
conditions which fortunately will require a certain number 
of years — say 1880 to 1885 — for their complete unfoldment ; 
hence their action may not be wholly manifest in a special 
month of any year • but this whole cycle of years is liable 
to be affected by a generally disturbed condition of the 
earth and its inhabitants.' 

But utter rubbish as all this is — the offspring of sheer 
ignorance and hysteric vapours — it is not much more absurd 
than the prediction recently based on the observed fact that 
the comet of 1880 travelled along the same path as that of 
1843, this path lying very close indeed to the sun. As- 
suming, as is really not improbable, that the comet of 1843 
passed so near to the sun as to have been retarded by the 
resistance of the corona, and so came back after a shorter 
circuit than it had before traversed, it is likely enough that 
the comet will next return after a yet shorter interval. 
Possibly Marth's period — ' say seventeen years,' he puts it — 
may be nearer the truth, in which case the comet would 
come back in 1897. The next return after that might be 
in seven or eight years, say in 1904. The next perhaps is 
three or four, and very likely by about the year 1920 or 
1925 that comet may reach the end of its career, being 
finally absorbed by the sun. It is also very likely that if, 
instead of being thus gradually checked off, so to speak, 
this comet in its original full-sized condition, with many 
millions of millions of meteoric attendants, had rushed full 
tilt upon the sun, it might have done a deal of mischief. A 
very able astronomer, Professor Kirkwood, of Bloomington, 
Indiana, believes (and very likely he is right) that two of 
the larger meteoric attendants on this comet falling into the 
sun in September 1859, produced that remarkable solar dis- 
turbance which was accompanied by very remarkable mag- 

R 



242 MYSTERIES OF TIME AND SPACE. 

netic disturbances and auroral displays all over the earth : so 
that doubtless the whole comet with its attendants pouring all 
at once upon the sun would have stirred him in a way which 
we should have found very noteworthy, even if we did not 
find it absolutely destructive to the earth and its inhabitants. 
But as a mere matter of fact (and so counting for something, 
whatever end-of-the-world prophets may imagine) the comet 
of 1843 and 1880 does not travel full tilt upon the sun, and 
can never do so ; its meteoric attendants are not all 
gathered in a single cluster, but form an immensely long 
train (if Kirkwood was right in the above quoted surmise, 
those which fell into the sun in 1859 were at least sixteen 
years behind the main body) ; and it is clear that a very 
effective interruption of the comet's career in 1843, repeated 
in 1880, can take place without in any appreciable degree 
affecting our comfort, still less our existence. If the comet 
of 1880 was the same object as the object of 1843, it showed 
very evident signs of having suffered grievously during its 
former perihelion passage. If it is proportionately reduced 
at its next return, we might even see it fall straight upon 
the sun (were that possible) without much fearing any evil 
consequences. Nothing which is known about comets in 
general, or about this comet in particular, suggests the 
slightest danger to the solar system, though everything 
suggests that the comet's career as an independent body 
will before very long come to an end. If the comet ever 
was a dangerous one, owing to the concentration of its 
meteoric components, it is not so now. If it really has been 
effectively checked in its career, it is evident such interrup- 
tion can take place without harming us, and therefore the 
final throes of the comet need not trouble us in the least. 
If it has not been effectively interrupted, then the end is 
not nearer — in any appreciable degree — now than it was 
in 1843 or in 1668. In any case, the end of this comet's 
career, whether far off or near at hand, will in all probability 
take place in such a way that terrestrial astronomers will 
never know of the event. 



243 



THE MENACING COMET. 

In 1881 a dismal report appeared, to the effect that the 
comet of 1843, which was supposed to have returned in 
1880, would come back again in 1895 and bring about 
the end of the world. The origin of the report was not 
altogether clear. At least it was not altogether clear to the 
writer of these lines, who, if the report had had any legiti- 
mate foundation, should have known something about it. It 
seems that a remark to the effect that the comet of 1880 
travelled in the same orbit as the comet of 1843, an d was 
probably the same body, but that if that were the case, it 
had returned long before it should have done, so that the 
period of revolution seemed to be shortening, had been to 
some degree misapprehended. 

It had been suggested by several Fellows of the Royal 
Astronomical Society that if the comet of 1880 were really 
the same as that of 1843, the next return might occur in a 
very few years; perhaps, said Mr. Marth, in about fifteen ; 
and each return thereafter at shorter and ever shorter in- 
tervals. For the path of the comet carries it in very close 
proximity to the orb of the sun; and it is generally believed 
that a retardation of the comet's motion must occur at each 
return to the sun's neighbourhood, for the simple reason 
that the comet can hardly be supposed to get through the 
matter which forms the sun's corona, without encountering 
some resistance. The more the comet is retarded by such 
resistance, the faster it will travel round its orbit — paradoxi- 
cal though this may sound. At each return it will encounter 



244 MYSTERIES OF TIME AND SPACE. 

more and more effective resistance, until at length it must 
be absorbed into the body of the sun. 

Whether such absorption would produce any great effect 
or not upon the sun, and through him upon the solar system, 
was a question which to many seemed answerable only in 
one way. Newton had pointed out that comets might serve 
as fuel to the sun, and perhaps produce disastrous effects in 
that way, by unduly increasing the solar light and heat. ' A 
comet,' he said, ' after certain revolutions, by coming nearer 
and nearer to the sun, would have all its volatile parts con- 
densed, and become a matter fit to recruit and replenish the 
sun (which must waste by the constant light and heat it 
emits) as a faggot would this fire if put into it.' (He was 
speaking to Mr. Conduitt at the time, beside a wood fire.) 
' And that would probably be the effect of the comet of 1680 
sooner or later ; for by the observations made upon it, it 
seemed to have a tail of thirty or forty degrees, when it went 
from the sun. It might, perhaps, make five or six revolu- 
tions more first ; but whenever it did, it would so much in- 
crease the heat of the sun, that this earth would be burnt, 
and no animals in it could live/ ' He took the three phe- 
nomena seen by Hipparchus, Tycho Brahe, and Kepler's 
disciples,' he added, ' to have been of this kind ; for he 
could not otherwise account for an extraordinary light, as 
those were, appearing all at once amongst the fixed stars (all 
which he took to be suns enlightening other planets, as our 
sun does ours) as big as Mercury or Venus seems to us, and 
gradually diminishing for sixteen months and then sinking 
into nothing.' 

But although what we now know respecting the mass of 
comets is by no means so much opposed to these views as 
many seem to imagine, our knowledge of the way in which 
the sun's heat is maintained will not permit us to adopt 
Newton's opinion. Nor will the accepted views as to the 
origin of the sun's heat justify us in accepting a belief in 
more than a very moderate accession of heat as likely to 
accrue, under any influences due to comets now actually 



THE MENACING COMET. 245 

travelling around the sun. All those which have passed 
once round the sun's immediate neighbourhood, can pass 
again, and yet again, with effects which can never greatly 
exceed those produced at their first passage. If at any one 
perihelion passage a comet is slightly retarded, it will be 
slightly retarded again at its next passage close by the sun, 
somewhat more at the next return, and so on continually, 
until it is finally absorbed, the interval between these pas- 
sages continually diminishing. Only in the case of great 
retardation at one passage, will the retardation at the next 
perihelion passage be markedly greater ; but in this case 
the effects at the earlier passage should have been note- 
worthy ; so that as no noteworthy sudden accession of solar 
light and heat has ever been observed, no such earlier pas- 
sage has yet occurred which should make us seriously fear 
the next passage of the same comet by the sun's neighbour- 
hood. 

The fears entertained, therefore, respecting the next 
return of the comet of 1843 are without foundation. If 
that comet was really so checked in speed in 1843 tnat ft 
returned in thirty-seven years instead of the much longer 
period assigned to it by the best astronomers, then we had 
an opportunity at that time of estimating the effect of such 
interruption of the comet's motion. But no effects were 
then perceived. The sun was neither brighter nor hotter than 
usual. The inference is, then, that that frictional resistance 
cannot appreciably affect the sun's condition. In 1880 we 
had a repetition of this experience — assuming that the comet 
of 1880 was the same body. The sun in 1880 shone much 
as he had done in 1879, much as he did later in 1881 and 
1882. So that the world might await with calmness the 
future returns of this sunlashing comet, satisfied that what- 
ever effect might be produced on the comet, very little 
would be produced on the sun or the solar system. 

But now suddenly news comes that a comet has been 
seen which American men of science have identified with 
the comet of 1843 and 1880, so that from thirty-seven years 



246 MYSTERIES OF TIME AND SPACE, 

the period has dwindled to little more than two years and a 
half (more exactly 2 years, 7 months, and 2 1 days), which 
would leave us every reason for believing that the next 
return would occur in a few months, and the final absorption 
of the comet by the sun a few weeks later. And an English 
astronomer of deserved repute has done something more 
than- endorse these ill-omened predictions; he has pretty 
clearly indicated his opinion that the approaching destruc- 
tion of the comet portends events of the most serious import 
to this earth and all who dwell on it ; that, in fact, the 
time is drawing near when Prospero's prediction is to be 
fulfilled that— 

The great globe itself, 
Yea, all which it inherit, shall dissolve, 
And like an insubstantial pageant faded, 
Leave not a rack behind. 

1 Could there have been anything more heartbreaking to 
all astronomical souls/ writes Professor Piazzi Smyth, Astro- 
nomer Royal for Scotland, 'than the uninterrupted cloud by 
day and by night of our unfortunate climate, ever since the 
announcement of the brilliant daylight comet of Monday, 
September 18? Tuesday, Wednesday, Thursday, Friday, 
Saturday, and their several nights, have each and all been 
uniformly utterly covered in with thick impenetrable clouds. 
And yet we ought to confess that one other thing might have 
occurred even so as to make that cloudy appearance more 
aggravating, more grievously disappointing still. That one 
overtopping culmination of misfortune would have been' — 
if the comet had been announced as approaching instead 
of receding. 

It will be seen that the. Astronomer Royal for Scotland 
regards the comet in question as a rather important body. It 
is not an everyday comet whose approach is so important that 
failing to see it must be regarded as an ' overtopping culmi- 
nation of misfortune.' Now this comet seems to be none 
other than that comet. The body, or collection of bodies 
(for so rather must a comet now be regarded), which was 



7 HE MENACING COMET. 247 

visible to the naked eye on September 18 close to the sun— 
' a yard or so from the sun/ writes one startled observer — is 
no other than the comet of 1843, whose tail stretched half 
across the heavens, and which — like the comet of last month 
— was seen in full daylight; nay, even ' close by the sun.' 

Rightly to apprehend the significance of this portent, as 
viewed by Professor Smyth and many others, chiefly — unlike 
him — unscientific persons, we should inform our readers that 
in this year, according to the prophecies symbolically indi- 
cated in the Great Pyramid, the end of the dispensation 
'which began 1882 years ago is in some way as yet unknown 
to be brought about. Some celestial body, ' the star in the 
East ' of the Magi, appeared then : for aught we know it may 
have been the same comet, and the Wise Men of the East 
saw in it evidence that a new dispensation was about to 
begin. It was fitting, then, that this year, which has now 
been for several years announced as the time of the end of 
that dispensation, a similar celestial appearance, or the same 
body, perhaps, should announce ' the beginning of the end.' 
We cannot reasonably doubt this, for careful measurement 
shows that the Grand Gallery in the Great Pyramid is 1,882 
inches long ; these inches being each the twenty-fifth part 
of the sacred cubit, which Pyramidalists assure us is the limit 
of length in that marvellous structure. Moreover, it was 
not altogether an accident or a mere coincidence which 
brought the British army to the feet of the Great Pyramid at 
the very time — perhaps at the very hour — when the great 
comet was passing its perihelion. On September 13 the 
British cavalry entered Cairo ; on September 18 the great 
comet could be seen with the naked eye (though it had 
passed the time of its greatest splendour, described by Pro- 
fessor Smyth as the ' ecstatic display at perihelion passage '), 
and was then beginning to recede. What more natural than 
to suppose that as the vanguard of Sir Garnet Wolseley's 
army approached the base of the Pyramids, the great comet 
was in the very ecstasy of perihelion glory, rushing through 
the richest portion of the sun's coronal streamers, molten by 



248 MYSTERIES OF TIME AND SPACE. 

the solar heat, resisted by the densely aggregated meteor- 
streams, but so retarded that its return will be hastened, and 
that in a few months it will come back to effect the final 
purpose of its existence ! If any doubt could be entertained 
on the subject, it should be removed by the consideration 
that the British nation has been proved, to the satisfaction 
of nearly all true believers in the Great Pyramid prophecies, 
to be no other than the lost ten tribes of Israel. 

If this sounds a little strange — or, shall we say, the least 
little bit premature — let the following words by the Astro- 
nomer Royal for Scotland, by no means the least able of our' 
astronomers, and facile princeps among Pyramidalists, be 
carefully considered. 

'What comet,' he asks, 'was this? The little that was 
seen on Monday, September 18, is not enough to give any 
clue, and no London journals, whether scientific or political, 
which I have seen up to September 23, throw any light on 
the matter. But a note by cable from America, if fully cor- 
rect, is of profound import. Indeed, nothing so important to 
all mankind has occurred before i through eighteen hundred 
years at least of astronomical history. And there is this pro- 
spect of the statement being true, that it is given under the 
name of Professor Lewis Boss, one of the most able and 
learned mathematical astronomers of the Union, and, we 
may say now (such has been the rapid progress of astronomy 
during the last few years in that country), of the world. He is 
said, then, to have concluded from his observations that the 
comet of last Monday was the comet of 1880 and 1843. A 
comet on each of these occasions was recognised to have 
passed closer to the sun's surface than any other known 
comet. But why has it come back so soon? In 1843 it 
appeared to be moving in an orbit of 170 years, and yet it 
came back in 1880, or in only 37 years. That was startling 
enough, though only looked on by the world as a case of 
failure of astronomical, prediction. But having gone off in 
1880 on an understanding generally come to by the best 
astronomers in Europe, North America, Rio Janeiro, the 



THE MENACING COMET. 249 

Argentine Republic, and Australia — at all which latter places 
it had been well observed — that it was not to return before 37 
years (and other comets, such as Halley's, and Encke's, keep 
to their times of revolution round the sun nearly uniformly 
for centuries), behold this comet has returned now, on the 
strength of this cablegram from America, in two years. 
In which case, who can say whether it may not be back again 
from space in a few months ; and then, not merely to graze 
close past, but actually to fall into the sun, which is so evi- 
dently increasing its hold upon it at every revolution? 
Wherefore we may be near upon the time for witnessing 
what effects will be produced when such an event takes 
place in the solar system, as astronomers have hitherto only 
distantly speculated upon, and no mortal eye is known to 
have ever beheld.' 

This brings the matter home to all of us, indeed. Astro- 
nomers like Newton have distantly speculated upon the 
effects which would be produced if a comet fell into the 
sun. I fear that I have not altogether refrained from such 
speculations myself. Indeed, the misapprehension to which 
I referred at the beginning of this paper arose chiefly from 
such speculations of my own. For speaking, not of such 
grazing contact as may occur in the case of the comet of 
1843 and 1880, but of such direct impact as may through 
some unlucky chance occur in the case of some comet which 
comes to our sun from interstellar space, I have expressed 
the opinion that such impact may raise the sun's heat tem- 
porarily to such intensity that every living thing on this earth 
would be destroyed, though the increase of heat might not 
last more than a few weeks or even days. I also expressed 
my belief (entertained before I had heard that Sir Isaac 
Newton, in conversation with Mr. Conduitt, had expressed 
similar views) that the appearance of so-called ' new stars ' 
can only be explained by the downfall of meteoric and 
cometic matter upon some sun like our own, which up to 
that time had been steadily pouring forth heat and light to 
nourish the worlds circling around it. This opinion, chancing 



250 MYSTERIES OF TIME AND SPACE. 

to be expressed in the closing paragraph of the same paper 
in which I had indicated my belief that the comet of 1880 
really was the same body as the comet of 1843, returned 
before its time, and that this body would return next after 
a yet shorter interval, led many to imagine that I had 
expressed the opinion that the comet of 1843 and 1880, re- 
turning soon, would cause our sun to blaze out with greatly 
increased splendour, and so to destroy all living creatures on 
this earth. 

Now the actual risk from the destruction of this comet 
by the sun I believe to be very small indeed. But as to the 
identity of the comet which passed its perihelion on Septem- 
ber 17 last and the comets of 1880 and 1843 there is, I 
think, little room for doubt. I have carefully compared the 
observed positions on September 17, 18, 19, 22, 24, and 29, 
with the known orbit of the comet of 1843, and they all 
agree so closely as to leave no doubt that the new cornet is 
travelling in the same track, so far as the part near the sun 
is concerned. But I note one circumstance which seems 
hitherto to have escaped attention. Although the course of 
the new comet as it passed away was on the right track, the 
comet was not making nearly so much way as it should have 
done, if moving in the reduced period of 2§ years, or even 
in one year, or in half a year. In other words, the reduction 
of speed experienced by the comet last September was such 
that the comet will be back within four or five months, pos- 
sibly in less time still than that. It may be that the ob- 
servations (up to the day of my writing this, which of course 
precedes by several weeks the day when these words can be 
read) have been insufficiently exact for accuracy in this 
respect. But if they can be trusted, the comet will be back 
in a very short time indeed, possibly before the end of the 
year. [It was soon found that the comet will not return 
for many years.] 

Be this as it may, it is certain that the splendid comet 
seen on September 18 and 19 close to the noonday sun, 
although not seen under conditions at all favourable to 



THE MENACING COMET 251 

ordinary observation, gave of all the comets seen in this 
century, nay, of all ever seen by man, the fullest promise 
that one day cometic mysteries will be interpreted. An 
observation was made upon this comet successfully, which, 
repeated on similar comets more favourably situated, will 
give information such as astronomers have long regarded as 
essential to the solution of cometic mysteries, but such also 
as they have hitherto scarce dared to hope for. 

It is of course known to all who have followed the pro- 
gress of recent scientific research, that nearly all the comets 
which have been observed during the last score or so of 
years, have given under spectroscopic analysis such evidence 
as shows that a portion of their light comes from glowing 
gas. Two distinct cometic spectra have been observed — 
Dr. Huggins, facile firinceps among British spectroscopists, 
first noted them in the case of Brorsen's comet, and of 
Winnecke's — each consisting of bright bands. In one case 
the bands have not been identified with those belonging to 
any known terrestrial substance ; but the other and more 
common cometic spectrum agrees with one which has 
been found to be characteristic of certain compounds of 
carbon. ' The general close agreement in all cases,' writes 
Dr. Huggins, ' notwithstanding some small divergencies, of 
the bright bands in the cometary light with those seen in 
the spectra of hydrocarbons, justifies us fully in ascribing 
the original light of these comets to matter which contains 
carbon in combination with hydrogen. 

These spectra of bands had been seen so systematically 
from 1864, when Donati made the first rough observations 
of the cometic spectrum, until Wells's comet was observed 
a few months ago, that astronomers began to think that 
they would get no other information from comets. It was 
especially unsatisfactory that no bright- or dark lines could 
be seen. For in the case of one particular class of spectro- 
scopic observations, which seemed specially likely to give 
interesting information about comets, bright bands in the 
spectrum are absolutely useless. We refer to those obser- 



^m 



252 MYSTERIES OF TIME AND SPACE, 

vations which indicate rapid motions of recession or 
approach, by displacements of the spectrum. Such dis- 
placement is always exceedingly small even in the case of 
bodies moving at the rate of twenty or thirty miles per 
second. It therefore cannot possibly be determined by 
observing a spectrum of broad bands of light, with no well- 
defined edges by which to recognise displacement. 

But Wells's comet in the spring of 188 1, though it attracted 
no special attention from ordinary star-gazers, showed for the 
first time a new and promising feature. This comet, which 
had shown the carbon bands like other comets during the 
first month or two of its approach towards the sun after its 
discovery, began, when it drew within a certain distance 
from him, to show evidence of the presence of glowing 
sodium. A few days later the pair of orange lines in the 
spectrum which indicate the presence of this widely distri- 
buted element, were very bright and distinct, and they con- 
tinued so until the comet passed out of view from our 
northern heavens. 

Now there was double promise in this observation. 
First, it showed that the changes of appearance which a 
comet undergoes as it draws nearer to the sun are accom- 
panied by changes of physical condition with which the 
spectroscope can deal. Secondly, as the bright lines of 
sodium are well-defined, and as their proper place in the 
spectrum is known, there was promise that hereafter obser- 
vations might be made to determine movements of reces- 
sion or of approach which may be taking place either in 
different parts of the comet, or in the comet as a whole. 

Let us first consider the application of spectrum analysis 
to determine the changes taking place in the physical con- 
dition of a comet. 

It is obviously a most promising circumstance that evi- 
dence should now be attainable to show what is the real 
physical constitution of those different parts of a comet 
which present such striking changes as the comet approaches 
the sun. Hitherto all that has been seen has been the 



THE MENACING COMET. 253 

raising up of luminous envelopes on the side towards the 
sun, and the apparent sweeping away of the matter thus 
formed into the strange appendage called the tail. But 
hereafter, in the case of any comet which like Donati's (in 
1858) exhibits under favourable conditions the various 
changes due to the increased proximity of a comet to the 
sun, it will be found possible to recognise by means of the 
spectroscope the substances which are successively volati- 
lised as the comet moves towards the perihelion. It may 
possibly be found that when a comet shows, as Donati's did, 
several envelopes one within the other, the luminous vapours 
forming these are of different substances. The constitution 
of the tail, too, may be found to vary as the comet changes 
in position. Where there are more tails than one, as in the 
case of Donati's comet, and of other celebrated comets, the 
spectroscope may indicate varieties of physical structure and 
condition. Possibly, Bredichin's theory, that three different 
substances — iron, carbon, and hydrogen — are driven from 
the sun with different velocities, for the several tails of such 
comets, may be established by the spectroscopic analysis of 
these appendages. It may very probably be found, also, that 
even in the case of a comet with but a single tail, the physical 
constitution of the tail varies in different parts of its length. 

But the possibility that movements in the nucleus, 
coma, and tail of a comet may be detected by spectroscopic 
analysis, is yet fuller of promise. 

Let us briefly consider the nature of this method of 
observation. 

When we approach a point from which waves of any 
sort are moving, we cross the waves in more rapid succes- 
sion, and the effect is as though they were narrowed. When, 
on the other hand, we recede from their source, so that the 
waves (moving, it is understood, more quickly than we do), 
overtake us, they pass us in less rapid succession, and the 
effect is as though they were made broader. (We speak, of 
course, of their width as measured from crest to crest.) We 
can easily see that this would be so in a sea across which 



254 MYSTERIES OF TIME AND SPACE. 

waves were swiftly travelling, a stout swimmer urging his 
way so as either to meet them or to be overtaken by them. 
It has been shown, also, experimentally that this is true of 
sound. When we approach a source of sound, the tone is 
raised (or rather appears to the ear to be so, for, of course, 
the sound-waves on which the tone depends are not really 
altered), whereas when we recede from the source of sound 
the tone seems lowered. This observation, indeed, may 
readily be made by any observant person in railway travel- 
ling ; for it will be noticed that whenever the whistle of a 
passing engine is sounded the tone falls suddenly, or seems 
to do so, at the moment when the engine which had been 
approaching begins (having passed him) to recede. In the 
case of light, it was long since pointed out by Doppler that 
a similar effect should be produced, if only the velocity 
of approach or recession is not too small to be appreciable 
when compared with the tremendous velocity of light — 
186,000 miles per second. The effect would theoretically 
be a change of colours in the case of light really of a single 
pure colour. For light belonging to the red end of the 
spectrum is formed by waves of greater length than those 
which form light belonging to the violet end of the spec- 
trum ; and the various colours of the spectrum from the red 
to the violet end have wave-lengths gradually diminishing 
from the greatest length at the red end to the least length at 
the violet. Doppler was bold enough to hope that by this 
method the colours of the stars might indicate stellar move- 
ments of recession or of approach. But of that he should 
have seen, had he reasoned the matter aright, there was no 
hope or even possibility. For the light of a star contains 
rays of all colours from red to violet, and rays beyond the 
red on one side, and beyond the violet on the other, which 
therefore no eye can see. The only effect of any diminu- 
tion of all the wave-lengths would be that a part of the 
violet light would be lost as light, but its place would be 
taken by light from the indigo, that by light from the blue, 
and so on, the light from the red which became orange 



THE MENACING COMET. 255 

being replaced by rays otherwise invisible from beyond the 
red. And similarly (only the change would be in the other 
direction) in the case of an increase in all the wave-lengths. 
But it was early shown (so far as I know, I was the first 
to refer to the matter publicly, but Dr. Huggins — unknown 
to me — was working at the very time on the plan indicated) 
that the lines in the spectrum would be shifted — towards 
the red in the case of recession from the source of light, 
and towards the violet in the case of approach towards 
that source. This displacement can be measured — if great 
enough, or rather, if not too small ; for, in the case of all 
such motions as are taking place among the stars and planets, 
the displacement must be very, very small indeed. 

Now to comets more than to any other class of celestial 
bodies this method might, it would seem, be advantageously 
applied. For not only do comets themselves move during 
a part of their course around the sun with enormous velocity, 
but within the comet itself changes take place which seem 
to imply enormously rapid motions. In particular the 
development of the tail, although it has not been absolutely 
demonstrated to be due to repulsive action, yet seems expli- 
cable in no other way ; and if it is thus caused, the move- 
ment of the matter forming the tail must take place with a 
velocity bringing it well within the application of the spec- 
troscopic method. 

But it is essential for the use of this method that the 
spectrum of the moving body should have well-defined and 
recognisable lines. Bands, such as those in the spectrum 
of the comets first observed, are utterly useless for this 
purpose. Their precise position cannot be determined so 
that we could be sure of any displacement due to motion. 
For this purpose we must have a line, which, when the spec- 
trum is brought side by side with that of a terrestrial sub- 
stance showing the same line, will be in line with this if the 
celestial source of light is at rest, and will be recognisably 
displaced towards the red or towards the blue if that lumi- 
nous body is receding or approaching respectively. 



256 



MYSTERIES OF TIME AND SPACE. 



So that when, last May, Wells's comet suddenly began 
to show the well-known lines of sodium, promise was at 
once, and for the first time, afforded, that the problems of 
cometic changes, in so far as these depend on motions taking 
place within the comet itself, may before long be solved. 
We can have very little doubt, for instance, that if such a 
comet as Donati's were now to appear, and to be studied 
under favourable conditions during those parts of its course 
in which it was subject to the most intense disturbing action, 
the bright lines which would be seen in the comet's spec- 
trum would either by their displacement tell us that the 
substance of the comet is driven wildly hither and thither in 
the head and swept swiftly away to form the tail, as it seems 
to be, or else, by remaining unchanged in position, would 
show that there are no such movements of disturbance or 
repulsion. 

Now the comet which has recently been seen near the 
sun has been observed by this method. On September 18, 
when it was but three degrees (say half-a-dozen sun-breadths) 
from the sun on the sky, it was examined in the clear sky of 
Nice by M. Thollon, a skilful French spectroscopist. 

The spectrum, notwithstanding the obviously unfavour- 
able conditions under which the observation was made, 
showed clearly the line (or rather the double line) of sodium. 
Here, by the way, was at once evidence such as in former 
times no astronomer could have of the comet's real position 
in space. Formerly if a comet was observed anywhere, once 
only, nothing could be certainly known respecting its posi- 
tion, except that it was somewhere in the line of sight in 
which it was seen. But if we are right in believing that the 
sodium in a comet is only vaporised and rendered self- 
luminous when the comet is near the sun, then the new 
comet on September 18 Was not only shown to lie in a 
certain direction, but within certain tolerably narrow limits 
of distance. 

But Thollon observed something else, not quite so satis- 
factorily as to be absolutely certain of it, but still so as to 



THE MENACING COMET. 257 

give a considerable degree of assurance. He says that the 
line of sodium seemed displaced towards the red. This 
would indicate recession. Observe here again how the 
spectroscopic method of determining motions of recession 
or approach may come in to help the astronomer to deter- 
mine the position of a comet. Supposing this method should 
ever be so improved that the exact rate of a comet's motion 
might be determined by it, then instead of merely ascer- 
taining, in any single observation, the direction in which a 
comet lies at the moment, the astronomer may learn its 
direction, something (as we .have seen) of its distance, and 
the rate at which it is moving from or towards the observer. 
The rate of its thwart motion cannot of course be inferred 
from the spectroscopic observations directly, yet indirectly 
it can. For the rate of motion at any given distance from 
the sun for an orbit of known dimensions is known ; now 
the distance of the comet being partly indicated by the 
spectroscopic observations, the thwart motion is known 
within the same degree of error. Hence, combining this 
with our more precise knowledge of the motion of recession 
or approach, we make a first rather rough approximation to 
the real motion, both in direction and in amount — which 
would determine the orbit absolutely. Observations made 
a day or two later will show whether the body really is 
moving in this orbit ; and if the later observations include 
spectroscopic ones we shall obtain means of testing and 
correcting the first estimate of the orbit, which will practi- 
cally give us the orbit correctly — much more correctly, at any 
rate, than it can be deduced by the methods at present in use 
from observations made on four or five different occasions. 1 

It may be well, perhaps, in conclusion, to inquire how 
the comet would actually be absorbed by the sun. 

First, then, be it noticed that at first there would be no 
tendency towards a diminution of the perihelion distance 

1 Theoretically the orbit of a comet can be deduced from three 
observations ; but practically many observations are required to give 
anything like accuracy. 



258 MYSTERIES OF TIME AND SPACE. 

of the comet, as many seem to imagine. The point of 
nearest approach must remain nearly at the same distance 
from the sun at each return of the comet, so long as the 
orbit remains eccentric. Only when the velocity in peri- 
helion (or at the point of nearest approach) is so reduced 
that the centrifugal tendency no longer balances the centri- 
petal force, would there be any approach towards the sun.. 
This amounts to saying that until the orbit is transformed 
into a circle (when there will be no perihelion at all) there 
will be no approach towards the sun. When that trans- 
formation is effected, there will be approach at every part 
of the circuit — in other words, the course of the comet will 
become a spiral, the coils of which will draw closer and 
closer in towards the sun's surface : the sun will be within 
the coils, but the comet itself will be in the toils, and its end 
not far off. As throughout this approach the comet's sub- 
stance would be in the form of vapour, there would probably 
be a rapidly increasing resistance, and hence a rapidly increas- 
ing rate of approach towards the sun. Oddly enough, the 
comet's rate of travelling would be increased notwithstanding 
this constant resistance, the sun's indrawing action adding 
more motion than the frictional resistance subtracts. For 
several days, probably, the comet in each circuit, when off 
the solar disc, would be a conspicuous object to spectro- 
scopists, though not perhaps visible through the telescope. 
The comet would appear outside of the sun's disc, first on one 
side, then on the other, at intervals of about if hours— 3^ 
hours being the time of circuit of a body close to the sun's sur- 
face. As this surface is carried round once in about twenty- 
five hours, there would be considerable loss of velocity, and 
resulting heat, in the substance of each part of the comet as 
it was absorbed. But I believe the whole heat of the sun 
would be little increased if the whole of the comet were thus 
absorbed at once ; and very little indeed if, as is certain, the 
absorption took place piecemeal. 



259 



JUPITER S SATELLITES. 

Jupiter surpasses our earth more than 1,400 times in 
volume. Saturn alone can be compared with him in this 
respect, but even Saturn is but half as large as Jupiter. In 
mass, this superb planet is not merely ' facile princeps,' but 
exceeds much more than twofold all the other planets taken 
together. We may view, indeed, in Jupiter and his system, 
a miniature, but instead of being a miniature of our earth it 
is as a miniature of the whole solar system that he is to 
be regarded. The sun himself does not so greatly exceed 
Jupiter in volume as Jupiter does our earth. And the bodies 
which circle round Jupiter travel with velocities comparable 
with those of the swiftest members of the solar system. 
While Mercury and Venus travel 100,000 and 80,000 miles 
an hour, and our earth travels 68,000 miles an hour round 
the sun, Jupiter's inner satellite travels upwards of 40,000 
miles an hour around its primary. Mars travels 55,000 miles 
an hour round the sun ; the second satellite travels 32,000 
miles an hour round Jupiter. Jupiter himself sweeps less 
swiftly round the sun than these satellites do around him, 
so that through a portion of their orbits they are actually 
retrograding. The third satellite also travels so swiftly 
round Jupiter as to be reduced very nearly to. absolute 
rest when its velocity acts in a contrary direction to that of 
Jupiter. The fourth satellite travels less swiftly than the 
third, but yet as swiftly as the planet Saturn in his orbit 
around the sun. 

Nearly every celestial object has an interest attaching 



260 



MYSTERIES OF TIME AND SPACE. 



to it other than that derived from its physical aspect— an 
interest which may be called historical. In the moon, for 
instance, we see an object without which (it is not too much 
to say) astronomy would never have approached its present 
state of exactness and accuracy. Mars, in like manner, 
afforded evidence such as no other planet could supply, 
when Kepler was engaged in the series of researches which 
rendered his name illustrious, and without which Newton's 
views might never have been directed to gravitation as a 
universal principle. Venus is connected with the deter- 
mination of the fundamental element of all astronomical 
measures — the sun's distance from the earth. Mercury, 
Saturn, Uranus, and Neptune, the sun, fixed stars, comets, 
asteroids, and nebulae, all have their historical interest, de- 
rived from the evidence which they have afforded on special 
questions of inquiry. Jupiter is second to none in this 
respect. At a critical period in the history of astronomy, 
when the world of science was divided on the subject of 
the Copernican Theory of the Universe, and when all with- 
out the world of science were steadfastly opposed to the 
new views, the discovery that Jupiter was the centre of a 
miniature system, circling around him as the theory in dis- 
pute taught that the planets circled around the sun, came 
opportunely as an illustration, and to those who could grasp 
the significance of the phenomenon, as a proof, of the views 
of the German astronomer. Later came a yet more remark- 
able and important discovery, through the observation of 
Jupiter's system — the discovery that light does not travel, 
as had been supposed, instantaneously, but with a mea- 
surable, however inconceivable, velocity. Through this 
discovery, supplemented by Bradley's discovery of the 
aberration of the fixed stars, came a proof — which is abso- 
lutely beyond cavil or question — of the true theory of the 
solar system. Supplementary proofs of Newton's view r s have 
been derived also, as might be expected, from the influence 
exerted by a planet whose disturbing agency so largely 
exceeds that of all the other members of the solar system. 



JUPITER'S SATELLITES. 



261 



Let us return to Galileo's discovery of the satellite- 
system of Jupiter, and the influence of that discovery on 
the views of astronomers. It was immediately felt, by those 
who opposed the new views of Copernicus, that the dis- 
covery of Jupiter's moons was fatal to their objections. 
Accordingly they spared no efforts in casting doubts on the 
observations of Galileo. Some asserted that the Tuscan 
had seen no such sights as he pretended. Others, that he 
had indeed seen them, but in illusive dreams ; that he was 
the sport of demons specially sent to punish him for a 
prying, inquisitive, and truth-doubting spirit. 'We have 
looked,' they said, ' for hours through his telescope, and 
have seen no such sights as he and his friends have de- 
scribed.' When at length it was impossible to deny the 
existence of Jupiter's moons, it became the fashion to dis- 
pute the real character of their movements. It was argued 
that these objects do not revolve around the planet, but 
travel backwards and forwards behind its disc. Down to 
the middle of the seventeenth century many refused to 
believe that the satellites actually circulate around Jupiter. 1 

The discovery, by Cassini, in 1665, that the satellites 
can be traced when their orbital motions carry them be- 
tween the planet and the earth, placed the true character 
of these bodies beyond a doubt. By means of Campani's 
object-glasses of 100 and 136 feet focal length, Cassini was 
able to see the satellites projected as small bright spots on 
the disc of the planet. He found also that their motions 
when thus situated are precisely those due to an orbital 



1 For aught I know the motion of the satellites may be denied to 
the present day. In the preface to the last edition (1823) of the 
Principia, edited by the learned Jesuits Le Sueur and Jacquier, there 
occurs the following remarkable passage : 'In adopting the theory of 
the earth's motion, to explain Newton's propositions, we assume 
another character than our own, for we profess obedience to the 
decrees of the popes against the motion of the earth.' It is, therefore, 
not wholly impossible that decrees may have been promulgated against 
the circulation of Jupiter's satellites also. 



■*l 



262 



MYSTERIES OF TIME AND SPACE. 



motion around the planet, and therefore very different from 
those of bodies attached to the planet. This circumstance, 
and the fact that the bright spots remain unchanged in form 
as they pass over the disc, proved incontestably that he had 
not mistaken bright spots, such as are sometimes seen on 
the body of the planet itself, for the satellites whose ingress 
on the disc he had previously watched. But he was able to 
detect another evidence of the true nature of these bodies, 
since he discovered that the shadows which they cast upon 
the body of the planet are visible as small dark spots upon 
the disc. 

Forty years later Maraldi observed that the fourth 
satellite does not always present the same appearance as it 
traverses the disc of the planet. Sometimes it appeared to 
him as a bright spot, at others it appeared darker than the 
planet. He noticed also that when the satellite seemed to 
be projected as a dark spot, this spot was smaller than the 
shadow of the satellite. ' According to the laws of optics,' 
he says, and others have followed him in the statement, ' it 
ought to have appeared larger.' Assuming this view to be 
correct, and that the observations of Maraldi were rightly 
interpreted by him, we are led to a somewhat singular 
result. It seems to have been shown by Sir W. Herschel's 
observations, that all the satellites of Jupiter follow the law 
observed in the case of our own moon — turning constantly 
the same face towards their primary. He observed that 
each satellite varied in brightness in different parts of its 
orbit, but that when it arrived at the same position in its 
orbit, ' it exhibits always the same degree of brightness. It 
would follow from this, that each satellite in transiting the 
disc of Jupiter should exhibit invariably the same appearance 
— since when so situated we always see the same half of the 
satellite, that half namely which is invisible from Jupiter. 
This*, at least, would always happen, unless a satellite were 
subject to transient variations of brilliancy arising from 
physical change occurring on its own face. Maraldi's obser- 
vation would seem therefore to point to the occurrence of 



JUPITER'S SATELLITES. 



263 



such changes on the fourth satellite, and corresponding 
observations of variations of brilliancy in the other satellites, 
by Cassini, Maraldi, and Pound, would lead to the same 
conclusion as respects these bodies also. The observation 
by Bianchini, that in other parts of their orbits the satellites 
are subject to considerable variations of brilliancy, would 
seem to confirm this result. 

Now without asserting the impossibility that the above 
explanation is the true one, I cannot but consider that it is 
highly improbable that the satellites of Jupiter are actually 
subject to physical changes of the kind implied. The ob- 
servations of Sir W. Herschel are decidedly opposed to 
Bianchini's view, and scarcely less directly contradictory 
of Maraldi's. It appears to me far more probable that the 
apparent loss of brilliancy observed by Maraldi was relative 
only, and due to the projection of the satellite on a brighter 
part of Jupiter's disc (which we know to be subject to partial 
variations of brilliancy), than that the whole, or nearly the 
whole, hemisphere of a satellite should suffer change in 
the manner imagined. The fact that the satellite appears 
smaller than the shadow, so far from being contrary to the 
laws of optics, as many have supposed, is directly deducible 
from those laws. The black umbra should indeed be 
smaller, but the complete shadow formed of umbra and 
penumbra together, should be larger than the satellite. 

I may notice, in passing, that observations having refer- 
ence to the relative brilliancy of celestial objects are at all 
times difficult, but that those made towards the end of the 
seventeenth, and in the earlier part of the eighteenth cen- 
tury, appear specially unreliable. Whether from the use of 
unwieldy focal lengths, or from imperfection in the single 
object-glasses, or from a want of thorough appreciation of 
irregularities due to atmospheric causes, certain it is that 
there are recorded a multitude of observations of this sort; 
in the interval named, which have not been confirmed by 
subsequent observation. 

Soon after his discovery of Jupiter's satellites, Galileo 



■wi 



264 



MYSTERIES OF TIME AND SPACE. 



perceived the use to which the phenomena they presented 
might be applied for the determination of the longitude. 
He was sanguine indeed as to the use of this method for 
finding the longitude at sea, not being aware, it would seem, 
of the mechanical difficulties which render the method 
unavailable on shipboard. With the object of constructing 
tables of the satellites' motions, he observed them for many 
years. The tables he formed disappeared unaccountably 
on the death of his pupil Rimieri, to whom he had entrusted 
them for publication, and were accidentally discovered a few 
years ago in a private library at Rome. Notwithstanding 
the amount of labour bestowed upon them, the tables are 
far from representing with accuracy the motions of the 
satellites. Galileo, indeed, and those who followed him in 
attempting the work of tabulating these motions, altogether 
underrated the difficulty of the task. A long series of ob- 
servations by Hodierna, Borelli, Cassini, Maraldi, Bradley, 
and a host of other observers ; the rigid theoretical scrutiny 
of the subject by Newton, Walmsley, Euler Bailly, Lagrange, 
Laplace, and others \ and a laborious comparison of the re- 
sults of observation and theory, by Lalande, Wargentin, 
Delambre, and Woolhouse, have been required to bring the 
theory of the system to the exactness and accuracy it has 
now attained. 

The relations actually presented by the motions of the 
satellites are very singular. They are partly exhibited by the 
following table : — 



Sat. 


Sidereal revolution 


Same in seconds 


Sidereal motion 
per second 


Distance from 
Jupiter's centre 


d. h. m. s. 




// 


miles 


I 


I 18 27 33-505 


152853-505 


8-478706 


278,542 


2 


3 13 13 42-040 


306822-040 


4-223947 


442,904 


3 


7 3 42 33-36o 


618153-360 


2-096567 


706,714 


4 


16 16 32 11-271 


I44I93I-27I 


0-898795 


1,242,619 



JUPITER'S SATELLITES. 265 

It will be observed, at once, that the period of the 
second satellite is almost exactly double the period of the 
first, and the period of the third almost exactly double that 
of the second ; and, of course, a corresponding relation 
holds amongst the sidereal motions of these bodies. This 
of itself is remarkable, but far more singular is the relation 
which regulates the extent to which the above relations differ 
from exactness. It is to exhibit this that I have added the 
column of sidereal motions, because the relation in question 
is masked when the sidereal periods only are given. It will 
be found that the sidereal, motion of the first satellite, to- 
gether with twice the sidereal motion of the third, is exactly 
equal to three times the sidereal motion of the second 
satellite. Thus : — 

(8"-4787o6) + 2(2"-096567) = i2"-67i840 = 3 (4"-223947). 

To show the effect of this singular relation, suppose the 
first and third satellites to start from conjunction, then after 
four revolutions of the first satellite, the second has per- 
formed nearly one revolution, so that they are very nearly 
in conjunction again, but have in reality passed their con- 
junction by a small angle. At the actual moment of con- 
junction, the first has described three complete circum- 
ferences and an arc (A, suppose), which is nearly a complete 
circumference, while the third has described the arc A only ; 
thus twice the motion of the third satellite added to the 
motion of the first gives us three complete circumferences, 
and three times the arc A ; and therefore by the above 
relation the second satellite has moved through one com- 
plete circumference together with the arc A. Hence neg- 
lecting complete circumferences the actual change of position 
of each of the three satellites is the arc A, very nearly equal 
to a complete circumference. They therefore hold the same 
relative position at the end as at the beginning of the inter- 
val considered. Now nothing was said as to the position of 
the second satellite. As a matter of fact when the first and 
third satellites are in conjunction the second is always in, 
opposition to both. 



266 MYSTERIES OF TIME AND SPACE. 

Wargentin, who devoted a life to the examination of the 
motions of Jupiter's satellites, but who was no adept in the 
higher branches of mathematics, found as the result of obser- 
vation that the relation above described was so closely ap- 
proximated to, that 1,317,900 years would have to elapse 
before the three satellites could be in conjunction. This 
result affords an interesting measure of the accuracy of ob- 
servation up to Wargentin's day, since Laplace has shown 
that the relation is absolutely exact. Librations may take 
place on either side of the mean state (though the most 
careful modern observations exhibit no trace of such libra- 
tion), but there is no possibility of accumulative change, 
save by the influence of effective agencies external to the 
system. It is somewhat singular that the comet of 1767 
and 1779 Passed through the middle of Jupiter's system 
without producing any observable derangement of the mean 
motions of the satellites — a fact which proves conclusively 
that the mass of the comet must be small, its density incon- 
ceivably minute. 

In Ferguson's c Astronomy ' it is stated that the motion 
of the fourth satellite presents no approach to a relation of 
commensurability with those of the others. A simple re- 
lation exists, however, with a closeness of approximation 
which is quite remarkable. In fact, throughout the whole 
solar system there is no relation of commensurability which 
brings closely following conjunction-lines so near to each 
other as this does. 1 The relation is this : — three times the 
period of the fourth satellite is 5od. ih. 36m. 33*8135., and 
seven times the period of the third is 5od. ih, 57m. 53-5203.; 
the difference, 21m. 19707s., is less than one-ii23rd part of 
the period of the fourth satellite. Thus when the third 
satellite has travelled round seven times from a given con- 
junction-line with the fourth, the fourth has gone round three 

1 Since the above was written, I have found that some tables of 
elements of the Saturnian system give such periods to the satellites 
Dione and Enceladus as to produce a yet closer approach than that of 
the two sat tes of Jupiter whose motions are here discussed. 



JUPITER'S SATELLITES. 267 

times and in addition one- 1 1 23rd part of a circumference, 
that is less than 20', and the third overtakes the fourth be- 
fore the latter has passed over 15' more (since 15 : 35 :: 3 : 7). 
This conjunction-line then is separated from a preceding 
one (the fourth preceding) by less than 35'. The remark- 
able relation which causes the ' Great Inequality ' of Saturn 
and Jupiter brings neighbouring conjunction-lines nearly 8^° 
apart, a distance fourteen times as great as the above. 

From the connection between the motions of the first 
three satellites, it follows of course that the periods of the 
two inner satellites also approximate to commensurability 
with the period of the fourth. We have, in fact, fourteen 
revolutions of the second, or twenty- eight revolutions of the 
first, nearly equal to three revolutions of the fourth; but the 
approach is not so close as in the case of the third satellite. 

From the relation holding between the motions of the 
first three satellites it is impossible that all these bodies 
should be eclipsed at once ; but it will be readily seen that 
at regular intervals all three are in the same straight line 
with the planet's centre. If this happen when the sun (and 
therefore the earth, which with reference to Jupiter may 
always be considered to be close to the sun) is near the 
same line, these three satellites will be invisible, one or two 
being eclipsed, two or one (as the case may be) being pro- 
jected on Jupiter's disc. Such a phenomenon is not unfre- 
quently observable. 

That the fourth satellite may be hidden at the same time 
it must be nearly in a line with the other three. This 
relation is not often presented ; and, as already stated, the 
concurrence of this relation with the requisite configuration 
•as respects the sun and earth, is an occurrence very seldom 
to be observed. 

A circumstance that tends to render the simultaneous 
disappearance of the four satellites more uncommon than it 
otherwise would be, is the fact that the fourth satellite is not 
necessarily eclipsed or occulted at each conjunction with 
Jupiter. It may pass above or below his disc or shadow. 



268 MYSTERIES OF TIME AND SPACE. 

In fact this happens on an average in more than one -third 
of the revolutions of this satellite. This is ascribed by Sir 
J. Herschel to the greater inclination of his orbit ; but this 
is not the correct explanation. In fact the inclination of 
the fourth satellite is at present less than that of any of the 
others, and the mean value of its inclination is always less 
than that of the others. The true reason why this satellite 
so often escapes eclipse is its greater distance from Jupiter. 

It is commonly stated that the third satellite cannot 
possibly escape eclipse or occultation as it passes behind its 
primary, and must necessarily transit Jupiter's disc when 
passing before the planet. I find, however, that it is just 
possible for the third satellite to pass clear of Jupiter's disc 
in the latter case. A conjunction of many favourable cir- 
cumstances is, however, required, and the phenomenon must 
be a very uncommon one— much more so, indeed, than 
the disappearance of all four satellites. It is necessary that 
Jupiter should be in opposition when not far from peri- 
helion, at which time it happens (and but for this the phe- 
nomenon could never take place) that the earth is at nearly 
her greatest distance north of the plane of Jupiter's orbit. 
The satellite's orbit must have its maximum inclination to 
Jupiter's orbit, and the satellite must also be at its greatest 
distance from the last-named plane. The other satellites 
must also be so situated that the third is at its maximum 
distance from Jupiter ; for it is noteworthy that although 
the orbits of the two interior satellites are described as cir- 
cular, and that of the third as of small eccentricity, yet these 
orbits have an ellipticity due to the mutual attractions of the 
satellites. This ellipticity is wholly different from the ellip- 
ticity of the planetary orbits. The former is centric, the 
latter eccentric, the sun being in the focus of each planetary 
eclipse, while Jupiter is at the centre of the ellipse traversed 
by the inner satellites. 

The figure shows the relative size of the four satellites, 
according to the best modern measurements, and the 
theoretical dimensions and character of their shadows. The 



JUPITER'S SATELLITES. 



269 



shadows have not, indeed, been seen as here shown, no 
telescope yet constructed having sufficed, it would seem, to 
exhibit the penumbral band surrounding the true shadow as 
a distinct feature. Yet it must not be supposed because the 
shadows are not seen as here shown that any reasonable 
doubt can exist as to their true character. The determination 
of the extent of the penumbra surrounding the true shadow 
is a matter of very simple calculation. Thus, in the case 



1st Satellite. 2nd Satellite. 



?rd Satellite. 



4th Satellite. 




Shadow of Shadow of 

tst Satellite. 2nd Satellite 



Shadow of 
4th Satellite. 



of the fourth satellite, we have, first, the distance of the 
satellite from Jupiter's surface equals 1,150,000 miles (in 
round numbers). Now the sun, as seen from Jupiter, or 
from any part of his system, subtends an angle of about 6'; 
and a moment's consideration will show that the width of the 
penumbral band is determined by supposing two lines to be 
drawn from a point on the satellite's limb to the planet (and 
therefore each 1,150,000 miles long), inclined to each other 
at an angle of 6', the chord joining the ends of these equal 
lines gives the required width (technically, the width is the 
chord of 6' to radius 1,150,000 miles). From mathematical 



270 



MYSTERIES OF TIME AND SPACE. 



tables it is readily found that this chord is about 2,000 miles 
long. This, then, is the width of the penumbra. If the sun 
were a point, the shadow would, of course, be exactly as large 
in appearance as the satellite itself, that is, would be a circle 
about 2,930 miles in diameter. Half the penumbral band 
lies within, half without such a circle. Hence, the diameter 
of the true shadow is only 930 miles, and the penumbral 
band around this being 2,000 miles wide, the diameter of 
the space covered by umbra and penumbra is 4,930 miles. 
It is easy to calculate in like manner the figures of the other 
four shadows. We may thus draw up the following little 
table :— 



Satellites 


Distance from 
Jupiter's surface 


Diameter 
in miles 


Width of pe- 
numbral band 


Diameter of 
true shadow 


Diam. of 
penumbra 




miles 




miles 


miles 


miles 


I. 


223,180 


2352 


390 


1962 


2742 


II. 


380,956 


2099 


665 


1434 


2764 


III. 


634,193 


3436 


I IOO 


2336 


4436 


IV. 


1,148,623 


2929 


2005 


924 


4934 



These are the dimensions presented in the figure. 

But now it is quite clear that although the actual extent 
of the penumbra must be such as is here shown, yet no tele- 
scope could possibly reveal the full extent of the penumbra, 
since its outer limit passes insensibly into the general illumi- 
nation of Jupiter's surface. And different telescopes will 
present the apparent dimensions of the shadow differently. 
It would, of course, be absolutely impossible to compare the 
efficiency of one telescope directly with that of another in 
this respect ; because no amount of practice in the use of 
the micrometer would enable an observer to estimate the 
apparent width of the shadow with the requisite exactness. 
But it is not difficult to compare the work of two telescopes 
in revealing the extent of Jupiter's shadows indirectly. For 
it is very obvious that a telescope which does not reveal 



JUPITER'S SATELLITES. 271 

faint shadows well, would exhibit the shadow of the fourth 
satellite as perceptibly smaller than that of the third ; whereas 
a telescope more efficient in this particular respect would 
show the shadow of the fourth satellite larger than that of 
the third. Or, even if only the fourth satellite and its 
shadow were transiting the disc, it is obvious that the former 
telescope would make the shadow seem smaller than the 
satellite, while the latter would make the shadow larger than 
the satellite. 

It may be remarked here, in passing, that a difference 
corresponding to that here discussed respecting the satellites' 
shadows exists in the nature of the shadow of Jupiter at 
the parts of the cone entered by the several satellites. Thus 
the disappearance of IV. takes a much longer time than that 
of I., not merely on account of the slow motion of IV. (as is 
commonly stated), but on account also of the much wider 
fringing of penumbra in the cross-section of the cone of 
shadow where IV. enters it, as compared with the fringe in 
the cross-section where I. enters. 



:272 MYSTERIES OF TIME AND SPACE. 



TERRESTRIAL MAGNETISM. 

The study of terrestrial magnetism is gradually assuming 
an importance which half a century ago few would have 
thought it could ever attain. It is being brought into 
intimate relationship with laws affecting not our own earth 
only, but the whole of the solar system. Indeed, interesting 
as are the bonds of union which Copernicus and Kepler 
and Newton have traced in the relations of our system, it 
would seem as though we were approaching the traces of a 
yet more wonderful law of association. We see the earth's 
magnetism responding to the solar influences, not merely in 
those rhythmic motions which belong to the periodic vari- 
ations, but in sudden thrills affecting the whole framework 
of our globe. The magnetic storms which are called into 
action by such solar disturbances as the one of September, 
1859 (witnessed by Messrs. Carrington and Hodgson), are, 
we may feel sure, not peculiar to our own earth. The other 
planets feel the same influence — not perhaps in exactly the 
same way, but according to the constitution and physical 
habitudes which respectively belong to them. So that one 
can scarcely conceive a subject of study at once more 
promising and more interesting than the science on which I 
now propose to make a few remarks. 

To deal fully with terrestrial magnetism would require 
very much more space than is now at my disposal. The 
mere history of the science would need a volume ; its 
relations to experimental physics would need another ; and 
one other at least would be required to treat of the theories 



TERRESTRIAL MAGNETISM. 273 

which are suggested by the interesting relations which have 
been discovered by modern observers. 

In this paper, then, I merely seek to make a few notes 
on certain points which have occurred to me in reading 
what Arago, Hansteen, Humboldt, Sabine, and others have 
said on the subject of terrestrial magnetism. There are 
certain peculiarities in the way in which the results of obser- 
vation have been dealt with, which I wish in particular to 
comment on. I think, too, that certain general results 
appear to follow from what has been discovered respecting 
the secular variations of the earth's magnetism, which have 
not yet (so far as I know) been very closely attended to. 

There are three features of the earth's magnetic action 
which are chiefly attended to by observers — the declination, 
the inclination, and the intensity. 

The declination is the angle at which the horizontal 
needle is inclined to the north-and-south line. The incli- 
nation is the angle at which the dipping needle is inclined 
to the horizon. The intensity is the magnitude of the force 
with which the needle seeks the position of rest. 

Now if we travelled over the whole surface of our earth, 
and carefully determined the declination, the inclination, 
and the intensity of the magnetic action at every point, we 
should be able to map down on a chart of the earth the 
relations thus presented to our notice. And speaking gene- 
rally we should have the following peculiarities to deal with : 

First, as to the declination. We should find that in 
certain regions the magnet's northern end was to the west 
of north, while in certain other regions the reverse was the 
case. If we marked in the boundary line between these 
regions, it is obvious that we should have traced in a line 
along which the needle would lie due north and south. 
This is what is termed a line of no declination. On charts 
of the earth's magnetic relations, the position of this line for 
about the middle of the present century is usually indicated. 
In some maps a set of lines used to be added, along each 
of which the magnetic needle had a definite declination. 

T 






274 MYSTERIES OF TIME AND SPACE. 

No such lines are now marked in, however. The reason 
why they are omitted is that they are very complex. But I 
wish to call particular attention to the omission, for this 
reason, that the same consideration which renders the lines 
of equal declination unnecessary in a chart of the earth's 
magnetism, the same reason which gives them a complexity 
altogether unmeaning so far as the cosmical relations of 
terrestrial magnetism are concerned, renders also the line of 
no declination worthy of much less consideration than it has 
received at the hands of many eminent physicists. I shall 
presently show why this is so. 

Secondly, as to the inclination. We should find as we 
travelled over the earth's surface, that the dipping needle 
tends to verticality at two nearly opposite points, one close 
to the Arctic and the other to the Antarctic circle. These 
are called the northern and southern inclination-poles, and 
must not be confounded with the intensity-poles presently 
to be mentioned. As we leave either inclination- pole the 
dipping needle leaves its vertical position, and gradually 
approaches the horizontal direction, until, along a curve 
lying midway between the two poles, the needle becomes 
exactly horizontal. This curve is called the magnetic incli- 
nation-equator. The present positions of the inclination- 
poles are indicated in magnetic maps. The inclination- 
equator is also indicated. The curves which run like 
parallels around the two poles in such maps indicate the 
curves along which the dipping needle has a definite incli- 
nation (different, of course, for each curve). Another series 
of lines, which intersect in the poles, or at least tend towards 
the poles, run so as to cross the inclination-parallels at right 
angles. But they illustrate, properly speaking, the pecu- 
liarities of declination. They show in what direction the 
horizontal needle points in different parts of the earth. 
But it is to be noticed that as they cross the real meridians 
at constantly varying angles they are in no way associated 
with the lines of equal declination described in the pre- 
ceding paragraph. 



TERRESTRIAL MAGNETISM. 275 

Lastly, as to the intensity. If we noticed, in every part 
of the earth's surface, the number of times the needle 
vibrated through its position of rest in a given interval (this 
number affording a very exact measure of the intensity of 
the magnetic directive action), we should find that along a 
•curve lying near to, but not absolutely coincident with the 
inclination-equator, the intensity has a minimum value. 
This curve is called the intensity-equator. Leaving it either 
towards the north or south, we should find the intensity 
gradually increasing. We should not, however, find this 
increase guiding us to a northern or southern intensity- 
pole ; but we should recognise two magnetic intensity-poles 
in each hemisphere. The positions of these are indicated 
in magnetic maps. The intensity-equator shows a certain 
resemblance to the inclination- equator, but the two equators 
do not coincide. 

Now in considering the various relations here presented, 
it is very important that we should decide which property 
of the magnetic needle should receive our chief attention. 

General Sabine considers that the intensity is the pri- 
mary quality of the magnet in all such inquiries as we are 
at present concerned with • that is, in all inquiries respect- 
ing the wider relations of terrestrial magnetism. He remarks 
that ' all that relates to the force has a more immediate 
bearing than what relates to the direction of the needle, 
either in the horizontal or vertical plane ; these planes, 
although necessarily used by us both in observation and 
discussion, not bearing in themselves any direct relation to 
magnetism.' 

Without denying the force of these remarks, I could yet 
venture to point out reasons why the force is a less suitable 
relation than the direction, where the question is one of 
mapping down the equator, parallels, and poles, which are 
to teach us something of the earth's magnetic action and its 
secular changes. 

I must premise that the declination and the inclination 
may be combined in a single indication — viz., the direction, 



276 MYSTERIES OF TIME AND SPACE. 

with a certain convenience so far as the general teachings of 
terrestrial magnetism are concerned ; but that if we are to 
select one or other of these elements of the direction as our 
special guide, it must clearly be the inclination. For a 
glance at a map of the earth's magnetic relations will show 
that over all the earth, except those regions which lie close 
to the poles, the declination has comparatively narrow limits 
of range, whereas the inclination varies from o° to 90 . 

Now the great point to be determined by the student of 
terrestrial magnetism is the situation of the true magnetic 
equator and poles. The existence of more poles than one 
in each hemisphere is clearly a peculiarity depending on the 
irregular conformation of the earth's frame, combined — in 
some way as yet unknown — with the external influences on 
which the primary facts of terrestrial magnetism depend. 
We cannot doubt that if the earth were a homogeneous 
sphere she would have a definite magnetic axis and a 
definite magnetic equator, which would be the same in 
position, whether we considered them with reference to the 
intensity or the directive power of the needle. And what 
we must do if we are to learn what position the magnetic 
axis and equator would have, but for terrestrial irregularities, 
is to select that relation for our guidance which is least 
likely to be affected by such irregularities. 

Now I shall be able, I think, to show that the inclination 
has a very great advantage over the intensity in this respect. 
The matter is very simple, and has as yet, as it seems to me, 
been very little considered. 

How do we determine the intensity-equator and the 
intensity-poles ? The answer is, by determining whether the 
intensity is a maximum or a minimum. Now, it is a well- 
known principle that variable quantities change slowly in 
the neighbourhood either of a maximum or of a minimum ; * 

1 We see an instance of this in the change of the sun's midday 
elevation near the time of midsummer and midwinter ; and many other 
equally familiar illustrations will occur to the leader. 



TERRESTRIAL MAGNETISM. 277 

so that if we have to determine the position either of a 
point where the intensity has a maximum or minimum 
value, we might easily make a very appreciable mistake, 
merely from the fact that there is so slow a rate of change 
near such a point, and that small irregularities due to local 
peculiarities may easily set us far astray. 

The fact, then, that the intensity-poles and the intensity- 
equator are found by the determination of a maximum or 
minimum is very unfavourable to the choice of this relation, 
intensity, as our guiding element. It is as though an astro- 
nomer were required to determine the seasons of the year, 
not by observing the sun's passage of the vernal or autumnal 
equinox (which is comparatively easy, because the sun 
crosses the equator at a considerable angle), but by noticing 
when the sun attained his greatest distance above or 
below the equator, which is not easy because his distance 
from the equator changes so very slowly at those times, 
when, in fact, his motion is all but parallel to the equator. 

Let us see if there is any difference when we consider 
the inclination. 

The inclination-poles and equator are determined by 
noticing where the dip is 90 and o° respectively. Now the 
dip changes through these values as sharply as it changes 
through any other values. In other words, a change of 
place on either side of the line of no dip, or on either side 
of the place of the vertical needle, produces quite as much 
change in the inclination as a similar change of place from 
any of the inclination-parallels. The fact is, that if we 
could travel from one of the inclination-poles to the 
inclination -equator, thence to the other pole, and so on 
until a complete circle of the earth had been made, we 
should find the dip changing from 90 down to o°, thence to 
90 , and so through o° to 90 again, and always at the same 
rate, precisely as the elevation of the pole of the heavens 
would change if we could make the circuit of a meridian. 

It seems to me that for these reasons the inclination of 
the magnetic needle is an element whose indications are far 



278 MYSTERIES OF TIME AND SPACE. 

less likely to mislead us than those of the intensity. In 
fact, it appears almost as unreasonable to be guided by the: 
intensity in determining the true magnetic equator, as to' 
endeavour to determine the position of the earth's equator 
by a reference to the distance of different points on the 
earth's surface from the centre of our globe. Doubtless, a, 
curve approaching the earth's equator pretty nearly might, 
with much labour, be determined in this way ; but the 
determination by a reference to the elevation of the celestial 
pole is not likely to be abandoned by astronomers on that 
account. 

Now, having determined that the directive power of the 
needle is the guide which can best lead us to the determina- 
tion of the true magnetic equator and poles, we must yet 
make some remarks on the two elements of the magnet's 
direction. 

A stress which appears to me altogether unreasonable 
has been laid on the position and motions of the line of no 
declination. In every work on terrestrial magnetism with 
which I am acquainted, this line is made the subject of a 
long disquisition. Humboldt, Arago, Hansteen, and Sabine 
discuss its present peculiarities, and the peculiarities of 
those motions by which it has reached its present position. 
In fact, in endeavouring to form a general idea of the laws 
which govern terrestrial magnetism, these and other eminent 
physicists have paid more attention to the motions and 
peculiarities of this line than to any other feature. 

I would ask whether the line of no declination really 
merits such close attention ; whether rather it is not, of all 
the curves associated with magnetic mapping, the one most 
likely to lead us astray. And to prepare the reader to pay 
closer attention to this point than in the face of so many 
eminent authorities he might care to do, I may just mention 
the fact that (whether the line of no declination is calculated 
or not to lead us astray) it actually has led physicists to 
form directly opposite conclusions on a matter of extreme 
simplicity. The most striking feature of the line is its rapid 



TERRESTRIAL MAGNETISM. 279 

change of position, and yet physicists cannot agree which 
way it is travelling. Arago says it is clearly travelling from 
east to west. On this Sabine remarks that quite obviously 
the line is travelling from west to east, and he quotes 
Hansteen's maps in confirmation ; but Humboldt, who had 
carefully studied Hansteen's maps, takes the same view as 
Arago. 

The real fact is, that the choice of ' the line of no decli- 
nation ' as a guide to the real history of magnetic changes, 
is as unfortunate as the choice of the intensity as a guide to 
the position of the true magnetic equator and poles ; and 
for much the same reason. The slightest irregularities due 
to the effects of the configuration of different parts of the 
earth's surface suffice to cause the line of no declination to 
assume the most complex and irregular figures. And again, 
the grand process of cosmical action which leads to 
a continual change in the position of the magnetic equa- 
tor, causes the true line of no declination (that is, the 
line along which, if the earth were a perfectly uniform 
globe, there would be no declination) to be continually 
subjected to new irregularities, which have no bearing 
whatever on the cosmical laws of terrestrial magnetism. 
Let me illustrate my meaning by two examples : — 
What is required, that a point should be on ' the line of no 
declination,' is that the magnetic direction-lines should there 
be running north and south. Now suppose that on a 
certain portion of the earth the direction-lines have the 
position of slant-waved lines only running north and south 
near their middle points. Then a curve through these 
points belongs to the line of no declination. But now con- 
ceive that, through some cause or other (as a change in the 
earth's condition in this neighbourhood), the ' lines are 
slightly twisted, so that at these points they do not run 
north and south but nearer the general direction of the 
waved lines. Then there are no points at all belonging to 
the line of no declination, within the area covered by the 
lines. Thus the portion which had crossed this area dis- 



28o MYSTERIES OF TIME AND SPACE, 

appears altogether, merely through the effects of a very- 
slight change. 

Next consider a case where a set of direction-lines have 
no points of inflection, but become north and south without 
change of curvature. Suppose that here, also, a slight 
change is produced on the position of the direction-lines 
by local irregularities, and that this change produces such a 
change of curvature where the convexity of the lines had 
touched the north and south lines. Then, in place of a 
single line, we should have three lines of no declination 
across this region of the earth. 

What reliance, then, can be placed on the indications 
of a curve which is liable to such remarkable changes of 
figure, through disturbances quite within the limits ascribable 
to the effects of local irregularities in the magnetic action ? 
For it is to be remembered that the cases I have cited are 
not instances carefully selected to exhibit, in a magnified 
form, the effects of local irregularity. The direction-lines 
can only assume a north-and-south position in one of the 
two ways considered ; that is, either by inflection or by 
presenting their convexity to the terrestrial meridians. 
Nor are such disturbances as I have considered by any 
means exceptional. On the contrary, everything tends to 
show that at every point of the earth's surface the main 
effects due to the earth's magnetic action are slightly modi- 
fied by local irregularities. If a plan had been wanted for 
losing sight of the main effects of magnetic action, no 
better one could have been thought of than that of selecting 
the ' line of no declination ' as a guide. One might as well 
attach importance to a line on the earth, showing where 
gravity acts in an exactly vertical direction. We know that, 
owing to local irregularities, we should find such a line to be 
looped and twisted into curves of the strangest complexity, 
but no one. has 'ever thought that those curves could teach 
us any great truths respecting gravity. 

The reader will gather from the above remarks that my 
chief aim in these notes is to draw attention to the more 



TERRESTRIAL MAGNETISM. 281 

marked characteristics of terrestrial magnetism. Any chart 
of the earth's magnetic relations will serve to illustrate 
what remains to be said ; but stereographic charts l of the 
northern and southern hemispheres are the best. The 
irregularly curved link with its companion loop, which 
forms the ' line of no declination,' may be left out of con- 
sideration. The inclination-equator, the series of lines and 
parallels belonging to the same element *of the magnetic 
action, and the poles towards which these lines tend, or 
around which they lie, are to be our principal guide. 

The magnetic poles are not directly opposite each other. 
A point directly opposite the estimated place of the south- 
ern magnetic pole (Ross was unable to reach this point) 
would lie to the west of the northern magnetic pole, and 
nearer the true pole of the earth. And it is worth noticing 
that many of the direction-lines around the northern pole 
seem to indicate a point further west as that to which they 
really tend. All those which cross the northern parts of the 
Pacific exhibit such a tendency in a very marked manner. 
This is a circumstance of which I shall presently have 
occasion to remind the reader. 

The magnetic equator exhibits a tolerably uniform sweep, 
rjut it is not a perfect circle. It will be noticed that where 

1 My reason for preferring this projection in the present instance 
is that it presents correctly all the angles at which the various lines on 
the earth's surface intersect each other. This is important in the case 
of magnetic charts. Otherwise I have no great liking for the stereo- 
graphic projection. In my Handbook of the Stars, I have pointed out 
the incorrectness of the view that this projection is suited for exhibiting 
the earth in hemispheres, remarking that however truly the projection 
exhibits small figures, it cannot (as alleged) exhibit continents without 
great distortion. A glance at Asia and at South America, as exhibited 
in such stereographic charts as I have suggested, will serve to show 
that this view is just. It is important, however, to notice one great 
advantage of the projection, for the purpose mentioned. All circles on 
the sphere are projected into circles, so that where we see the magnetic 
parallel somewhat oval in figure, we know that they really do depart 
from the circular form. 



282 MYSTERIES OF TIME AND SPACE. 

it crosses the ocean it seems to seek a more northern 
course. 

Next as to the parallels. It seems worth noticing that 
as a general rale the neighbourhood of the great continents 
seems to sway these curves away from the northern mag- 
netic pole. The parallels in fact assume in the northern 
hemisphere an oval figure, the direction of the major axis 
lying along a meridian through Asia and America. The 
way in which the parallel nearest the southern pole is- 
swayed northwards where it crosses the Atlantic Ocean is 
also well worth noticing. 

These irregularities are neither marked enough in their 
character, however, nor consistent enough in their indica- 
tions to detain us here. Nor need anything be said about 
the peculiarities of the magnetic direction-lines (or meridians,, 
as they have been somewhat strangely named). 

Now let us remember what these lines and parallels 
represent. When we see London and New York nearly on 
the same magnetic parallel, we learn that the inclination of 
the dipping needle is nearly the same in London as in New 
York. If we go south from either place the inclination 
diminishes, if we go north it increases. 

Suppose, then, we find, that as time passes the inclination 
in London or in New York is changing — what opinion are 
we to form ? We can arrive at no other conclusion than 
that the distance of these places from the magnetic pole 
is changing. And since London and New York are not 
changing their place on the globe, we conclude that the 
magnetic pole is in motion. 

But the inclination in London is changing, is in fact 
steadily diminishing. We can at once interpret this change. 
Since by travelling away from the magnetic pole, we dis- 
cover a continually diminishing inclination, England must 
be receding from the nearest magnetic pole ; in other words 
the northern magnetic pole is receding from England. We 
see at once that the magnetic pole must be travelling; 
westward. 



TERRESTRIAL MAGNETISM. 283. 

But now let us adopt a new mode of discussing this 
important secular variation. 

One of the most thoroughly established of all the pecu- 
liarities of terrestrial magnetism is the fact that the magnetic 
declination varies from time to time. In London and 
Paris the variations of the declination have been observed 
more carefully, and for a longer time, than at any other 
place. Let us inquire what we can learn from them. 

They may be briefly stated thus : — 

In London the needle pointed to the east of north 
before the year 1657, when it pointed due north. From 
that time the westerly declination gradually increased, until 
.the beginning of the present century, when the westerly 
motion was observed to flag. In 18 19, the greatest westerly 
declination was reached. At this time the needle pointed 
2/j.f degrees to the west of north. Since then the needle 
has been slowly travelling eastwards, and 
the westerly declination is now only some * 

17 degrees, the needle pointing now as in 

fig. 1. 

In Paris the needle pointed due north 
in 1663. Its subsequent motions have s 

closely resembled those of the London Fig. i. 

needle ; but the Paris needle ceased to move westwards as 
early as 181 7, and the greatest westerly declination attained 
by it was only 22^ degrees. 

Now we can easily interpret both these sets of move- 
ments. If we were to perplex ourselves by looking at the 
line of no declination, to see where the needle points due 
north, we might indeed go very far wrong. But if we seek 
for direction-lines, which have a general north and south 
direction, we see that these lines lie nearly on the great 
circle passing through the northern magnetic pole and the 
pole of the earth. It follows that at some time near the 
year 1657 the northern magnetic pole must have been on 
the meridian of Greenwich, or on the prolongation of this 
meridian forming the meridian 180 E. (or W.) of Greenwich. 




284 MYSTERIES OF TIME AND SPACE. 

In other words the magnetic pole must either have been 
directly between England and the real pole of the earth, or 
directly beyond the pole. 

To determine which of these positions the magnetic pole 

had is very easy. If this pole were between England and 

the real pole of the earth, it is obvious that the magnetic 

inclination in England would have 

had a maximum value. In the 

other case the inclination would 

have had a minimum value. But 

the inclination has been continually 

diminishing since 1657. It is now 

as shown in fig. 2. Hence it then. 

had a maximum value ; and the 

fig. 2. northern magnetic pole lay between 

England and the real pole of the earth. 

Also we can tell which way the magnetic pole was 
travelling. For before 1657 the declination was easterly, 
whereas afterwards it was westerly. Hence the magnetic 
pole must have travelled from east to west round the north 
pole of the earth. This agrees with our former result. 

We can learn also something about the rate at which 
the magnetic pole is travelling. 

Supposing the magnetic needle in the meridian of 
London in 1657, and Ross's estimate of the place of the 
magnetic pole to be appreciably correct, giving (in round 
figures) 95 W. of Greenwich for the longitude of the 
magnetic pole in 1833, we get a period of revolution of 

•VV X (1833— T657) years 
=tI x i 7& years=667 years about. 

Combining Ross's estimate with the Paris epoch, we get 
a period of 

|f x (1833— 1663) years 
== ft ( x I 7°) years -644 years about. 

The mean of these values is about 655 years, and I think 
there is very good reason for believing that the northern 






TERRESTRIAL MAGNETISM. 285, 

magnetic pole revolves around the north pole of the earth 
from east to west, in about this time. 

The southern magnetic pole must, of course, have the 
same period of rotation as the northern. Hence a portion 
of the discrepancy alluded to above, between the northern 
magnetic pole and the antipodes of the southern, as esti- 
mated by Ross, is accounted for ; because Captain Ross 
determined the position of the northern pole ten years 
before he endeavoured to reach the southern one, and in 
ten years the northern pole had rotated over a considerable 
arc towards the west. 

The magnetic equator must also vary in position. Its 
inclination to the real equator may perhaps be nearly 
constant, but its nodes must travel round from east to west, 
making a complete revolution in the same period as the 
poles. Such a motion has been actually observed. 

I am sensible that when we examine the changes of the 
declination and inclination, over the whole face of the 
earth, so far as they have hitherto been determined, we 
shall find many irregularities which seem, at first sight, not 
easily reconciled with the above views. But it will be 
found that it is only when weight is placed on the re- 
latively unsatisfactory indications already mentioned that 
these peculiarities become perplexing. Three circumstances 
which have been much dwelt upon as opposed to the 
existence of such a rotation of the magnetic poles as I have 
described remain to be mentioned as among the clearest 
evidences that that rotation is actually taking place. Over 
the Chinese empire, Humboldt says, the change in the 
magnetic declination has been very slow indeed for several 
centuries. In Jamaica, Sir J. Herschel notices the needle 
has pointed nearly due north for many years ; and lastly 
Arago notices a similar peculiarity in the case of South 
Australia. Now if we look at a chart of the earth's mag- 
netic relations, we shall see that the reason of these pe- 
culiarities is closely bound up with the rotation I have 
described. The northern magnetic pole in passing from the 



■■■■ 



286 MYSTERIES OF TIME AND SPACE. 

meridian of Greenwich to its present position, has passed 
the point where it has its greatest distance from the Chinese 
empire, and consequently the oscillation of the needle (by 
the reverse of what was inferred for the case of London) 
would be slowest during the centuries occupied by this 
passage. In Jamaica and Australia the neighbouring mag- 
netic pole lies nearly due north and due south respectively, 
and has lately passed the meridian of those places, so that 
here, as in England in the middle of the seventeenth 
century, the needle has for many years had a small decli- 
nation. 



287 



THE STAR-DEPTHS. 



During the last two years it chances that I have many- 
times given lectures on the stars, and, accordingly, it will not 
be wondered at that many suggestive questions have been 
addressed to me by thoughtful persons to whom the won- 
derful teachings of modern astronomy respecting the star- 
depths have been submitted. In particular — and this is 
one circumstance in which, as I conceive, the lecturer has 
an advantage over the author — I have had an opportunity of 
ascertaining the difficulties which have occurred to those 
who have heard the narrative of what astronomy has done. 
I propose in the present essay to consider some of the 
points which have thus been brought under my notice, 
feeling assured that a large proportion of those who study- 
astronomical subjects are likely to find an interest in the 
matters I am now to touch upon. 

I may remark, in the first instance, that misappre- 
hensions are somewhat widely prevalent as to the knowledge 
which astronomers have obtained respecting the distances 
and real magnitudes of the fixed stars. The announcement 
is received with astonishment, and some degree of disap- 
pointment, that astronomers have only estimated the dis- 
tance of a single star in the whole heavens in a manner which 
can be regarded as satisfactory ; while even that estimate 
is probably many hundreds of millions of miles in error. 
Many appear to imagine that it is in a sense discreditable 
to astronomy that no greater success has been achieved in 
this direction ; others suppose that the statement has its 



"■I 



288 MYSTERIES OF TIME AND SPACE. 

origin in doubts or objections peculiar to myself; while 
some recall the fact that in books of astronomy the esti- 
mated distances of ten or twelve stars are indicated, to- 
gether with a variety of elaborate calculations as to the 
average distances of stars of the various orders of apparent 
brightness, or (as it is technically termed) magnitude. 

It is well that it should be generally known how power- 
less astronomy at present is in this matter of the determina- 
tion of stellar distances. The astronomer can deal only 
with the actual conditions of a problem of this sort ; he 
cannot select the conditions for himself. The surveyor of 
a difficult region of the earth can determine heights and 
distances by a variety of methods ; he can undertake and 
execute a variety of more or less difficult operations • and 
where one process of survey does not satisfy him, he can 
try another, or several others. But the astronomer who 
wishes to determine the distance of a star, has one means 
only for obtaining that change of standpoint on which all 
such measurements must, in the long run, depend. And 
that means does not depend upon himself. It is not he who 
is to undertake the journey by which the change of stand- 
point is to be obtained, but the earth carrying him and his 
telescope along with her in her yearly orbital motion round 
the sun, shifts his standpoint for him. The case is even 
less under his control than the problem of determining the 
sun's distance, now so engrossing the attention of astro- 
nomers ; for there the astronomer may vary his standpoint 
more or less, according as he is more or less in earnest in 
seeking to master his difficulties. He may, on the one 
hand, seek the bleakest and most dismal northerly regions, 
as the Russians propose to do during the coming transit of 
Venus ; or, on the other, he may be careless in pushing his 
way as far southwards as possible, even as (it is to be feared) 
this country is likely to be careless, on that important occa- 
sion. But the measurer of a star's distance has no choice in 
the matter. He must be content to wait until the earth has 
carried him from one side of her path to the other, a span 



THE STAR-DEPTHS. 289 

wide enough, it should seem, to measure almost infinite 
distances, since the base line thus given is more than 1 80 
millions of miles in length. But it is all too narrow for 
measuring star-distances, and if the astronomer could 
choose he would require the orbital sweep of Saturn, 
Uranus, and more distant Neptune, to give him a really 
effective hold on the stupendous problem. Even then, 
however, he would be left powerless in the presence of all 
but a few hundred stars ; beyond these, the millions on 
millions known to astronomers would lie at distances which 
would laugh to scorn all means of measurement. In fine, 
to measure effectually the distances of the stars, the astro- 
nomer should be able to voyage with an interstellar comet, 
and live millions of years for every hour of his present life. 

To show how feeble is the actual hold of the astronomer 
on this great problem, let us consider what the base-line of 
180 millions of miles really represents, in the case even of the 
nearest of all the fixed stars. To the observer, when stationed 
at one end of this vast line, a star is seen necessarily in a 
different direction than to the same observer when (six 
months later) he is at the other end of that line. But the two 
direction-lines are so nearly parallel, even in the case of the 
nearest star, that if two lines were drawn as nearly parallel 
from a point, they would have to be extended to a distance 
of nearly two miles from that point before they would be a 
single inch apart. Or the matter may be presented in this 
way. We know how little the minute hand of a watch or clock 
changes in direction in a single second of time. Now the 
direction of a line from the nearest star in the heavens to 
the earth shifts during the half-year by less than the change 
of direction of the minute-hand of a watch or clock in the 
two-hundredth part of a second. This is as much as to say 
that if that star be regarded as the centre of a monstrous 
clock-face, and a line from the star to the earth as the 
minute-hand, then the extremity of that hand would move 
over 180 millions of miles in the two-hundredth part of a 
second of time. 



290 MYSTERIES OF TIME AND SPACE. 

Now if astronomers had merely to recognise this change 
of direction, the task must be a sufficiently arduous one. 
In fact, two centuries passed after the first use of telescopic 
measuring-means before the slightest signs of displacement 
could be recognised in any star in the heavens. The earth 
during all these years was circling on her orbit with its vast 
span of 1 80 millions of miles : and yet the star-sphere 
within which this motion was taking place remained 
seemingly as unchanged as though the earth had been 
absolutely at rest — and this although the most powerful- 
instrumental means invented by man were applied to de- 
tect even the minutest sign of motion. But, as a matter 
of fact, the astronomer has to accomplish a much more 
difficult task before the distance of a star can be ascer- 
tained. All that the mere recognition of displacement 
would imply would be that the star does not lie at a 
distance so vast as to reduce to absolute nothingness the 
effects of the earth's orbital motion. The displacement must 
not be recognised only, but measured, before a star's distance 
can be estimated. Thus I have spoken of the change of 
direction as nearly equal to the two-hundredth part of the 
motion of the minute-hand of a clock or watch in a single 
second of time. It is such a fact as this that has to be 
ascertained. The astronomer must be able to say that the 
change is, let us say, greater than the two-hundred-and- 
tenth part and less than the two-hundredth part of such 
motion of a minute-hand, or to make some other definite 
statement indicating the limits of error in the result of his 
observations. Then, and then only, the star's distance can 
be said to be estimated ; though even then, be it noticed, 
the amount of error in the determination of a star's distance 
remains necessarily enormous. In the imagined case the 
error is probably within the twentieth part of the star's actual 
distance, but the twentieth part of the distance of the nearest 
star is more than ten thousand times the sun's distance. 

After this it will not be regarded as very surprising that 
I reiterate the statement made above to the effect that the 



THE STAR-DEPTHS. 291 

distance of only one star has been determined satisfactorily. 
But I will now adduce independent evidence on this point. 
The nearest star in the whole heavens is the leading 
brilliant, Alpha, in the Centaur, a star not visible in our 
northern latitudes. Now the next in order of distance 
(always referring, of course, to the observations made thus 
far) is a faint star in the northern heavens, numbered 61 
in the constellation of the Swan. Until of late it has been 
judged that this star lies about three times as far away as 
Alpha in the Centaur, and this estimate has been for 
many years repeated in our text-books of astronomy. But 
recently a more careful and complete series of observations 
has been effected, with the unexpected result that, so far as 
these particular observations are concerned, the distance of 
this star must be set at twice instead of three times the dis- 
tance of Alpha in the Centaur. Of course, this result, though 
it must be regarded as the most satisfactory yet obtained, 
cannot be regarded as final. The mere fact that it differs 
from the result so long accepted shows how open the whole 
subject is to doubt. And then, when we consider what the 
difference between the two results actually amounts to, we 
see that the very errors in the attempts to determine star- 
distances belong to the same order as the distances them- 
selves. Here, in the case of the nearest but one of all the 
stars in the heavens, we find that one estimate of the star's 
distance exceeds another by a full distance of the nearest 
star, or by upwards of two hundred thousand times the 
sun's distance. After this, it appears to me that it would be 
preposterous to place reliance on the estimates of the dis- 
tances of stars yet farther away. Astronomers have made 
one sure step, and stumbled at the second. Can it be 
thought that in the steps beyond the second they have 
walked surely ? 

But so soon as we recognise the fact that of all the 
millions of stars, one only has had its distance satisfac- 
torily determined, while perhaps some ten or twelve lie 
at measurable distances, we see that all our conceptions 



292 MYSTERIES OF TIME AND SPACE. 

respecting the star-depths must be, to a great extent, hypo- 
thetical. On our ideas of star-distances depend our ideas 
of star-magnitudes. Or rather, we cannot even go so far 
as this ; since it would only be possible to estimate the real 
magnitudes of the stars if we could measure their apparent 
magnitude. But so enormous are the distances of all the 
stars, that even with the most powerful telescope the largest 
and brightest stars remain mere points of light. So that, in 
fact, all we can do to form an idea of the real magnitude of 
any star is to compare the quantity of light we receive from 
the star with that which we receive from our own star, the 
sun. This has been done, and the general result is im- 
portant and interesting. It is this : if our sun were placed 
at the distance of the nearest of all the stars, he would shine 
as a star of the second magnitude (just bordering, however, 
on the class of first-magnitude stars). Now all the obser- 
vations hitherto made indicate that the first-magnitude stars 
lie, on the average, fully five times as far from us as the, 
nearest star Alpha in the Centaur. At this distance, the 
sun would shine only as a star of the fifth magnitude — that 
is, only an order higher than the very faintest stars seen on 
a perfectly clear and dark night. The inference manifestly 
is, that our sun is very much smaller than the majority of 
the stars which form our constellations. 

I pass now to another point, on which many misappre- 
hensions prevail. I have frequently been asked what, in 
my opinion, is the actual form of our star-system regarded 
as a whole. It is known that various hypotheses have been 
formed on the subject, and my own writings sufficiently 
indicate that I hold opinions which are, at least to this 
extent, definite— that they are opposed to those commonly 
presented in our text-books of astronomy. But what is not 
uncommonly asked is, that I should indicate what is the 
actual figure I prefer to the cloven flat disc of our text- 
books. 

Now I think it will be tolerably clear, from what has 
been said respecting star-distances, that our ideas as to the 



THE STAR-DEPTHS. 293 

figure of our star-system must necessarily be hypothetical, 
even if not speculative. Since when we look at any star, 
save nine or ten out of the millions revealed by the tele- 
scope, we are in absolute ignorance as to the star's distance, 
we must necessarily be ignorant as to the shape of the 
system which those millions of stars form, when regarded as 
a whole. 

It will not be marvelled at, then, if my reply to the 
question just mentioned is, that in my opinion the figure of 
our star- system is unknown, and in all probability unknow- 
able. 

Where then, it may be asked, do my views respecting 
the sidereal universe distinctly clash with those commonly 
(at least until quite recent times) received amongst astro- 
nomers ? Or, do they merely differ in this respect — that I 
regard as unsupported by sufficient evidence what has com- 
monly been held to have been placed on a highly probable 
basis ? 

I will indicate as plainly as possible in what respect my 
views about the sidereal universe, and the methods of 
inquiry I have adopted or suggest for future use, differ from 
the views and methods of those who have preceded me in 
this department of research. To make the matter simpler I 
shall limit my remarks to the views and methods of Sir W. 
Herschel and the elder Struve, because these astronomers 
alone have advanced, or dealt with, wide and general con- 
siderations. 

Every reader of astronomical text-books is probably 
familiar with Sir W. Herschel's method of observation 
called star-gauging, and with the result which he obtained 
from it in 1785. Oddly enough— the mistake is however of 
a piece with others made in this matter — the paper of 1784, 
in which Herschel described the method, is selected by 
compilers in preference to the papers of 1785, in which he 
described his application of the method and its results. 
Struve, alone, so far as I know, has pointed out the ab- 
surdity of this process. As he justly remarks, the later 



m 



294 MYSTERIES OF TIME AND SPACE. 

paper supersedes the earlier altogether. Yet Arago, Guille- 
min, and all the French writers of astronomy, and of course 
their English followers, have complacently reproduced the 
preliminary statements of 1784, as though indicating Her- 
schel's conclusions from the method of star-gauging. 

The method of star-gauging assumes such a degree of 
uniformity in stellar distribution within our star system — 
assumed to have attainable limits— that the number of stars 
seen within any given field indicates fairly the extension of 
the system in the direction of that field. Taking fields equal 
in size and observing them with equal telescopic light- 
gathering power, we should have the same sort of estimate 
of extension that we should obtain of sea-depth by letting 
down a tube to the sea-bottom and weighing all the water 
inclosed within the tube. All that is necessary to make the 
estimate satisfactory is that we should really reach the 
limits of the system, and that the stars really should be 
spread with tolerable uniformity. The size of individual 
stars counts for nothing in this method, assuming only (as 
Herschel assumed) that a telescope powerful enough to 
reach easily to the limits of the system would show every 
star within the system ; in other words, that there are no 
very great discrepancies of magnitude. 

It was as the result of this process and no other that 
Herschel (following Wright, of Durham, in this respect) 
arrived at the well-known stratum theory of the sidereal 
system — or rather the cloven stratum theory. This theory, 
plainly expressed, is as follows : our sidereal system is flat- 
tened in shape, limited in extension, cloven into two strata 
through about one half of its extent, and star-strewn through- 
out its whole extent. The stars of the Milky Way, according 
to this theory, constituted the main part of the sidereal 
universe. 

When this theory was formed, Herschel had not met 
with even the simplest cases of unequal star-distribution ; 
he had not yet learned that such objects as binary stars 
exist in the universe. Struve has well pointed out that the 



THE STAR-DEPTHS. 295 

discovery of the physical association between certain double 
stars changed the whole aspect of the star system in Her- 
schel's eyes. From this time he recognised the fact that 
there are different orders of stars, and different systems of 
•star-grouping. 

Does it need proof that Herschel himself abandoned 
altogether, as well the principle of star-gauging as the results 
•to which it had led him ? It ought not, but as a matter of 
-fact it does require proof ; it requires reiterated proof ; it 
requires to be stated and re-stated, and demonstrated and 
re- demonstrated, until the writers of astronomical text-books 
shall begin to entertain the idea that possibly the theory 
they advance so confidently as Herschel's may have been 
doubted by Herschel as time went on. 

Here are his words in 1802 — they seem clear enough : 
' The stars we consider as insulated are also surrounded by 
a magnificent collection of innumerable stars called the 
Milky Way. For though our sun and all the stars we see 
may truly be said to be in the plane of the Milky Way, yet 
I am now convinced by a long inspection and continued 
examination of it, that the Milky Way itself consists of stars 
very differently scattered from those which are immediately 
about us.' 

I only select these words because they afford such simple 
and conclusive evidence of Herschel's change of opinion. 
There are even more convincing details, however, which, 
to be properly understood, require to be examined in their 
proper place in Herschel's series of papers. Herschei hot 
only said that the Milky Way was not what he had sup- 
posed ; but he stated what in his opinion was the actual 
shape and constitution of certain parts of the Milky Way. 
He found that certain of the brighter parts of the Milky 
Way were not regions of the sidereal system owing their 
apparent richness to their vast extension, but true clustering 
aggregations — not parts of an extended stratum, but globe- 
shaped. He says of these regions. ' we may, indeed, partly 
ascribe the increase both of brightness and of apparent com- 



296 MYSTERIES OF TIME AND SPACE. 

pression to a greater depth of the space which contains these 
stars ; but this will equally tend to show their clustering 
condition ■ for since the increase of brightness is gradual, 
the space containing the clustering stars must tend to a 
spherical form, if the gradual increase of brightness is to be 
explained by the situation of the stars.' 

In 1 81 1, he used the following expressions : 'When the 
novelty of the subject is considered, we cannot be surprised 
that many things, formerly taken for granted, should, on 
examination, prove to be different from what they were 
generally (that is, in a general sense, not by the generality of 
persons) but incautiously supposed to be. For instance, an 
equal scattering of the stars may be admitted in certain 
calculations ; but when we examine the Milky Way, or the 
closely compressed clusters of stars, this supposed equality 
of scattering must be given up.' 

Herschel did not substitute a new method of stellar 
observation, or indicate any definite new views respecting 
the sidereal universe, until a much later epoch in his 
career. 

It was in 181 7 that Herschel described his resolution-test, 
very commonly confounded with star-gauging, but quite 
distinct in principle. According to this plan the distance 
of any star-group was to be determined by the telescopic 
power necessary to resolve the group. To show the dis- 
tinction between this method and the other, it is only neces- 
sary to point out that in star-gauging, the same telescope, or 
at least the same telescopic power, was necessarily employed 
throughout the inquiry ; in the new method, the telescopic 
power was made variable, gradually increasing power being 
applied until a star-group was completely resolved, and the 
power required for this purpose was held to indicate the 
distance of the group. There was a partial return, in the 
adoption of this method, to the general idea of uniformity 
of distribution which had led Herschel to erroneous results 
in 1785 ; and in precise correspondence with this fault in 
the new method we recognise in the results obtained an 



THE STAR-DEPTHS. 297- 

obvious incorrectness, which would not have escaped the 
attention of Herschel in earlier years. In fact, I have not 
the slightest hesitation in saying that, in my belief, the cele- 
brated papers of 181 7 and 1818, despite their undoubted 
power, are yet characterised by signs of the advanced age 
which Herschel had attained. There is no want of power, 
but there is not the old elasticity of mind ; ' the grace and 
versatility of the man ' which had been so marked in former 
years, can no longer be recognised, except in the fact that 
views so noble should have been conceived at all, and a 
method so difficult carried out with something of the energy 
of Herschel's earlier days. It is not, I conceive, to be 
wondered at if. at the age of seventy-nine, the great 
astronomer should not display those qualities of mind 
which specially characterised him in youth and middle 
age ; and it should not be regarded as indicating any 
want of respect for the memory of Herschel to point out 
in what circumstances these latest papers of his appear 
wanting. 

I would note, then, that the new method was one to be 
tested, not adopted. The method of star-gauging had been 
adopted, and had eventually to be rejected. This experience 
should have suggested that no new method ought to be 
adopted until it has been subjected to some adequate test. 
The results obtained by Herschel showed sufficiently (and 
in his earlier years would have been regarded as showing) 
that the method was unsound. 

Let one point suffice to show this : — 

The fine double-cluster in the sword-hand of Perseus 
was partly resolved with the lowest power Herschel em- 
ployed, and this, according to the criterion he had adopted, 
showed that part of the cluster lay only just beyond the 
limits of naked-eye vision. But he was unable with the 
highest powers he employed to resolve this spot completely^ 
and this, according to his criterion, showed that part of the 
cluster lay beyond the range of his highest powers, or some 
twenty times farther away than the limits of naked- eye 



398 MYSTERIES OF TIME AND SPACE. 

vision. Thus, according to his criterion, the distances of 
the nearest and farthest parts of the cluster are as i to 20, 
at least. Bat the apparent size of the cluster shows that the 
breadth of any part of the cluster is less than the hundredth 
part of the distance of that part. Combining 
these indications, we have for the shape ot the 
space within which the cluster is included some- 
thing like a b (fig. 1), where s is the place of the 
solar system. 

But no one can accept so monstrous a result 
as this; nor would Herschel for a moment have 
adopted it, had he noticed that his own figures 
led directly to it. Apart from all other objec- 
tions, the circumstance that this long projecting 
cluster had the sun directly at its apex, would 
have sufficed to show that some other inter- 
pretation must be adopted. But no one who 
has observed this or any other star-cluster can 
doubt for an instant that the portion of space 
occupied by the cluster is rounded — speaking 
roughly. The cluster is not a globular cluster, 
of course ; it has outlying branches and streams 
(and appears, by the way, to be connected with 
the general stream of the Milky Way in this 
neighbourhood) ; but that, regarding it as a 
whole, we must consider it as a rounded nodule 
or double nodule of the galactic stream, and not 
as a projecting region directed exactly from the 
sun, no one can doubt who has ever examined 
the cluster. 

Nor did Herschel question this, or fail to 
recognise the soundness of the method of reason- 
ing, illustrated by Fig. 1, which I have used to 
dispose of the strange result to which Herschel's 
figures lead. But the real fact is, that Herschel 
did not live to examine fully the facts he supposed he had 
accumulated in and after 181 7. He did not do much, 



THE STAR-DEPTHS. 299 

indeed, towards carrying out the scheme which he had 
indicated. The papers of 181 7 and 181 8 may be compared 
respectively with those of 1784 and 1785 ; and I venture to 
say, that if in 1818 Herschel had had before him as long an 
-observing career as in 1785, we should have found him 
definitely abandoning the principle he indicated in 181 7, 
precisely as we find him in 1802 definitely abandoning the 
principle he indicated in 1784. 

The fact is, both principles were matters to be tested, 
not to be adopted as rules for guidance. 

William Struve adopted a singular combination of the 
two methods, and interpreted the results he obtained in a 
most remarkable, and, as I think, altogether unsatisfactory, 
manner. 

Struve perceived that if the principle of star-gauging had 
been sound, the stars of the brighter orders ought to be 
strewn with general uniformity over the heavens ; for the 
range of naked-eye vision, or low telescopic power, for 
separate stars lies far within the limits of the stellar system, 
even in its narrowest parts — that is, measured in the direc- 
tion of its thickness. He saw then that here was a new test 
applicable to the star-gauging principle. It was only neces- 
sary to take some complete catalogue of stars down to, say, 
the 7th or 8th magnitude, and to try whether the stars grew 
richer as the Milky Way was approached. If they did grow 
richer, then it would be manifest that the principle of star- 
gauging could not be trusted in all„ its generality. (The 
reader will presently see why I emphasise these four last 
words.) 

Struve took a catalogue complete only for an equatorial 
zone 30 wide, and extending 15 from the equator on 
the north and on the south. The catalogue was tolerably 
complete down to the ninth magnitude inclusive. Like 
other catalogues, it was numbered and arranged in order of 
Right Ascension ; and what Struve did was simply to 
ascertain how many stars it included in the different ' hours.' 
He found that the Milky Way 'hours,' the seventh and 



300 MYSTERIES OF TIME AND SPACE. 

nineteenth, were the richest, and that there was a gradual 1 
increase up to the maximum of richness, and then a graduall 
decrease down to the minimum ; so that there was all the 
evidence his method could afford to show that the nine 
brighter orders of stars do increase in numbers as the Milky 
Way is approached. 

If he had been satisfied with this result, Struve would! 
simply have obtained rough evidence of those laws of stellar 
distribution which my methods have more particularly- 
indicated. 

But he was not so satisfied, and it was in analysing his 
result that he applied the novel considerations I have- 
spoken of. 

He first assumed that the law of distribution for an> 
equatorial zone 30 wide might be regarded as the law off 




distribution for the circuit of the equator itself — a particu- 
larly daring assumption, when we remember that the galaxy 
does not cross the equator at right angles but slantwise, so 
that an 'hour' of stars 30 in declination range crosses parts 
of the star-sphere of very different richness (assuming, as- 
Struve did, that the mid-zone of the galaxy is the zone of 
greatest richness). This first step, then — this substitution 
of the equator itself for an equatorial zone covering more 
than a quarter of the whole heavens — was directly calculated,, 
it appears to me, to mask the real features of stellar distri^ 
bution. 

But the next step was even more daring. Having found 
that there is a variation of star-richness round the equator,, 
two opposite points of which circle mark the place of maxi- 



THE STAR-DEPTHS. 301 

mum richness, Struve next changed the circle into a disc, 
and in this way : — 

Suppose a b, fig. 2, to be one ' hour ' of the celestial 
•equator, s the place of the solar system. Then he took the 
stars which he had distributed uniformly along the arc a e, 
and spread them (according to the numerical distribution 
of their different orders of brightness) over the sectorial 
.area abs. 

Surely an amazing application of the laws of averages, 
to take the stars spread over the area abed of the celestial 
sphere, where a b is an arc of 15 and a b, c d each of 30 , 
and to conceive these stars spread, according to a special 




law of uniformity, over the sectorial plane A b s (fig. 3). Yet 
it was thus that Struve's celebrated section was obtained. 

I hesitate to point out objections against this process 
because I cannot conceive what arguments can possibly be 
urged in its favour. 

Struve himself considered that his result agreed perfectly 
with the views of Herschel in 1817 and 181 8 ; and it will 
be perceived that there is something in the formation of the 
sectorial star- area s a b, which bears a distant resemblance 
to Herschel's resolution test of star- distance. On the other 
hand, Struve perceived that his results were altogether 
opposed to the general theory of star-gauging. He adopted, 



302 MYSTERIES OF TIME AND SPACE. 

however, a modified form of the principle, since he assumed 
a certain very peculiar law of uniformity of distribution. 

[In passing, I may remark that, in answering the first 
paper I read on this subject at the Royal Astronomical 
Society, Professor Pritchard made the somewhat remark- 
able statement that all Herschel's results confirmed each 
other, and that Struve's agreed with them !] 

The plan on which I have proceeded has been unlike 
any of those hitherto described, and the principle I have 
adopted is not an assumption, but one whose justice de- 
pends on manifest and sufficient considerations. 

I have, in the first place, set equal surface charting — as 
the only effective way of indicating laws of distribution, if 
any such exist — and charting generally as a necessary sub- 
stitute for mere cataloguing, or other process of numerical 
estimation, whether as applied to the stars of various orders, 
to nebulas, to clusters, to double, coloured, or variable stars, 
or, lastly, to star-motions. You can see a chart, study its 
relations, return again and again to its investigation, com- 
pare it with other charts, or combine it with them in a new 
chart, and so on. Any characteristics which charts are 
competent to exhibit, they show to all, and reveal inde- 
pendently of any special process. The case is altogether 
different with catalogues. Their capacity for concealing 
the truths really contained in them, and for mystifying the 
student of nature, and their adaptability for deceptive pro- 
cesses of manipulation, have been repeatedly illustrated in 
scientific researches. 

But of course, when once a chart has been drawn, it 
suggests questions for statistical research ; or rather it sug- 
gests really effective methods of dealing with numerical 
relations. And the construction of charts of distribution 
has led me to notice two principles of analysis, which are 
manifestly sound, and have already been shown to be 
effective, while I believe very confidently that they will 
hereafter be found to be the only principles which can 
enable us to make any satisfactory advance in dealing with 



THE STAR-DEPTHS. 303. 

those portions of space which lie beyond our means of 
measurement. 

They are these : — ■ 

A. If objects of one class are found to be spread over 
certain parts of the celestial sphere more richly than over 
others, and if objects of another class are found to exhibit 
corresponding peculiarities of distribution, being always 
more richly spread or more sparsely strewn where the same 
is observed of objects of the former class, then the two classes 
of objects form one system, and are intermixed within that 
system, the same subordmate region of space within that sys- 
tem including the aggregations of both classes of objects. 

B. If, on the contrary, a law of contrast is observed in 
the distribution of objects of two different classes, so that 
the distribution of the objects of one class is systematically 
richer or poorer according as the distribution of objects of 
the other class is poorer or richer ; then the two orders of 
objects form one system, but are separated from each other 
within that system, those subordinate regions of space which 
include many objects of one class having few or no objects 
of the other class, and vice versa. 

I will illustrate these principles before passing on to a 
third, which is only probable, not as these are certain (where 
only the evidence described can be obtained in sufficient 
amount). 

Suppose the whole sky were covered more or less 
richly with flying creatures ; suppose it could be seen that 
there were two well-marked classes of these creatures, large 
ones and small ones : then if, wherever the large Ones were 
richly strewn over the sky, the small ones were likewise so 
strewn, while where there were few large ones there were 
few small ones, it would be a safe inference that the large 
and small ones were intermixed in the aerial spaces, travel- 
ling together in flights or groups. They might be old birds 
and their small youngsters, or birds pursuing their prey 
among flights of insects, or the like ; but whatever the two 
classes might be, it would be certain (supposing always that 



304 MYSTERIES OF TIME AND SPACE. 

the peculiarity in question were sufficiently marked) that 
the large and small objects were intermixed. 

But suppose the reverse held, and that where there 
were many large flying creatures, there were few or no small 
ones, and vice versa, the peculiarity being very marked. 
Then it would be manifest that in this case also there was a 
law of association, but one which caused the objects of the 
two classes to separate into different groups. The large 
objects might be birds of prey, and the smaller might be 
weaker birds, banding together for mutual protection, or 
the like ; but it would be certain that the peculiar law of 
arrangement observed among the flying creatures was due 
to some real relation between the two classes, this relation 
causing them to keep apart from each other. 

In neither case — and let the reader notice specially, that 
what I arn about to say applies equally to both cases — in 
neither case could we imagine for a moment that the large 
flying creatures were of the same class as the small ones, 
but much nearer ; for it would be practically impossible 
that by some strange accident two strata of creatures, flying 
at different levels, should have their rich and poor regions 
so adjusted, as to be seen, in all cases, in the same direction, 
by the observer on earth. 

This leads me to the third principle, which only affords 
probable evidence, though, in some cases, such evidence 
may approach very nearly to demonstration. 

C. If objects of two classes are spread over certain parts 
of the heavens more richly than over others, but the rich 
regions of one agree systematically neither with the rich nor 
with the poor regions of the other, then the probability is, 
that the two orders of objects lie at different distances, and 
are in no way connected with each other. 

This probability rises into certainty where some markedly 
peculiar arrangement of one order is seen to be carried across 
some equally marked arrangement of the other, the two 
arrangements being manifestly independent of each other. 

The reader will perceive that the principles thus enun- 



THE STAR- DEPTHS. 305 

dated are those which I have followed in the interpretation 
of all my equal surface charts, whether of stars or nebulae. 
Applied to my chart of 324,000 stars, principle A shows 
conclusively that neither the earlier nor later view of Sir W. 
Herschel respecting the Milky Way, nor the theory of the 
elder Struve, is even an approach to the true theory ; but, 
the general views indicated by Sir W. Herschel during the 
period of his observing career, intermediate between the 
abandonment of the star-gauging principle and the adoption 
of the resolution-test, accord fairly with the observed facts. 
The point overlooked by him, by Struve, and by all other 
astronomers, except the younger Herschel (who, however, 
did not sufficiently dwell on its importance), is the evidence 
of the intermixture of stars of many orders of real magni- 
tude within the same regions of space. And as principle A, 
applied to the teachings of my chart of 324,000 stars, shows 
the Milky Way to consist of a system of star-streams, 
including many orders of stars, from the highest down to 
what must be regarded relatively as mere star-dust, so 
principle B, applied to my charts of Nebulae (and still more 
clearly when applied to the exact charts recently submitted 
by Mr. Sidney Walters to the Astronomical Society), shows 
that the star-cloudlets belong to the same system as the 
stars of the Galaxy, but are so distributed within that 
system as to be rich where stars are relatively few, and few 
where stars are relatively rich. 

But much wider and more systematic research is still 
required. It will be manifest that the three principles, but 
especially the first, admit of being applied most effectively 
to stellar research — much more effectively than star-gauging 
as originally proposed, even if it had turned out that the 
principle of such star-gauging was sound. By applying 
telescopes of different power to systematic star-gauging, as 
distinguished from the mere selection of a few widely 
scattered fields, and by employing for each survey only 
dark, clear, and moonless nights, instead of using (as was 
the case with Herschel's surveys) moonlight, twilight, and 

x 



306 MYSTERIES OF TIME AND SPACE. 

hazy nights, and even daylight, an estimate can be formed 
of the laws of distribution of the stars of various orders, 
proceeding onwards from those included in my chart of 
324,000 stars, and thus extending and amplifying the teach- 
ing commenced in that chart. When that work has been 
accomplished, we shall begin to understand the real wonders 
of the star-depths, the magnificence of the subordinate star- 
schemes which have been mistaken for the sidereal system 
itself, and something of the grandeur of that system, whose 
limits lie far beyond the range of our most powerful tele- 
scopes, while within them are included all the various 
orders of celestial objects which the telescope has yet 
revealed. Combining such lessons with what has been 
learned and yet remains to be learned of the movements 
taking place within the sidereal universe, we shall have a 
picture grander and more impressive by far than any yet 
presented to our contemplation ; we shall learn that the 
true Galaxy is infinitely more extended, infinitely more com- 
plex in structure, than we have supposed, and that the 
processes at work within its bounds are infinitely more 
stupendous. 



307 



TRANSITS OF VEXUS. 

During the autumn months of 1882 the evening star was 
seen drawing nearer and nearer, night after night, to the 
place of the sun, until at length she set too soon after him 
to be discernible save with the telescope. To the astrono- 
mer this approach of Venus to the sun had an interest 
greater than usually attaches to the phenomenon : for it was 
known that she would not pass from the eastern to the western 
side of the sun's orb without crossing his face. The passage 
of Venus close by the sun is, of course, a phenomenon of 
frequent occurrence and possessing no special interest. Hes- 
perus, the star of evening, cannot change into the morning 
star, Lucifer, without passing the sun's place upon the 
heavens ; nor can Lucifer change into Hesperus without a 
similar passage ; though there is a distinction between the 
two cases, for it is by passing between the sun and the earth 
that the evening star changes into the morning star, while it 
is by passing beyond the sun, so that the sun comes between 
the earth and her, that Venus changes from a morning star 
into an evening star. But these are astronomical pheno- 
mena which have been witnessed and understood for thou- 
sands of years. It is when Venus, in passing from the east 
to the west of the sun, does not steer clear of his disc, but 
traverses it, so that she appears in the telescope like a 
round black spot upon his face, that every astronomer is 
interested. For these occasions are few and far between. 
The last transit of Venus before 1874 occurred more than 
105 years earlier ; and although that transit was followed by 

x 2 



308 MYSTERIES OF TIME AND SPACE. 

another in 1882, yet after that second transit an even longer 
interval will elapse before another occurs, than has passed 
since the transit of 1769 : not until June 2004 will Venus 
again pass over the face of the sun. And besides the interest 
naturally attaching to a phenomenon which occurs so 
seldom, the transits of Venus have a scientific importance 
depending on their relation to celestial measurement, since 
they afford the best means astronomy possesses for deter- 
mining the distance of the sun, and with that distance the 
dimensions of the solar system, besides whatever information 
we may hope to possess respecting the tremendous distances 
which separate the sun from his fellow-suns, the stars. 

Transits of the inferior planets were, at first, only awaited 
with interest because of their bearing on the Copernican 
theory of the solar system. It was not until astronomers 
had abandoned the old systems that they could have any 
positive assurance that any planet ever passed between the 
earth and the sun. Moreover, regarding the celestial bodies 
as all self-luminous, astronomers could hardly have expected 
that, even if a planet passed across the sun's face, it would 
be discernible as a dark spot. We do, indeed, hear that 
some old observations of sun-spots were regarded as transits 
of Mercury or Venus across the sun's disc. Thus, the 
author of the ' Life of Charlemagne ' tells us that Mercury 
was seen in April 807, as a black spot upon the sun's face r 
for eight consecutive days. Kepler, who was perfectly well 
aware that Mercury could not remain as many hours on the 
sun's disc, endeavoured to show that the expression used in 
the manuscript of the old writer might not have been octo 
dies, but octoties, a barbaric form of orties for eight times. 
Again, the famous physician Ebn Roschd (commonly called 
Averroes) says, in his Ptolemaic Paraphrase, that in the year 
1 161 he saw Mercury on the sun at a time when the planet 
really was in inferior conjunction (that is, passing between 
the earth and the sun). Kepler himself believed that he 
' had so seen the planet. In his day Mercury was supposed to 
be a much larger body than it actually is. Hence there was 



TRANSITS OF VENUS. 309 

nothing surprising in the fact that an experienced astrono- 
mer like Kepler should have mistaken for the planet a sun- 
spot, seen no doubt only for a short time when the sun was 
low down. But later, when the telescope had revealed the 
existence of spots upon the sun's face, Kepler admitted that 
in all probability he had seen such a spot, and not Mercury. 
We know now that even Venus, much larger though she is 
than Mercury, and much nearer to the earth when in transit, 
is quite invisible to the unaided eye at such a time. 

Gassendi, who was the first to witness a transit of an in- 
ferior planet, saw Mercury pass across the face of the sun 
on November 7, 1631. His account of the observation is 
quoted somewhat fully in the Cornhill Magazine for Novem- 
ber 1868. Kepler, who had announced the transit, had also 
predicted a transit of Venus on December 6, 1631 ; and 
Gassendi hoped to witness this event. Kepler predicted 
that the transit would begin shortly before sunset ; but as 
the transit of Mercury had not occurred exactly at the time 
indicated by Kepler, Gassendi thought that quite possibly 
he might witness the whole of the transit of Venus. He was 
prevented from observing the sun on December 4 and 5 
by impetuous storms of wind and rain. ' On the 6th he 
continued to obtain occasional glimpses of the sun, till a 
little past three o'clock in the afternoon, but no indication 
of the planet could be discerned.' 'On the 7th be saw the 
sun during the whole forenoon, but looked in vain for any 
trace of the planet.' We now know that the transit took 
place during the night between December 6 and December 7. 1 

1 It is commonly stated that no part of the transit could have been 
witnessed in Europe. The present writer, however, having calculated 
the circumstances of the transit as accurately , as the case warrants, finds 
that the end of the transit could have been seen from the south-eastern 
parts of Europe, occurring at sunrise for all places on a line drawn 
from Gibraltar through Marseilles, Dresden, St. Petersburg, to the 
extreme north-east of European Russia. The account of Gassendi's 
failure in the excellent treatise Les Passages de Vemts, by M. Dubois, 
Naval Examiner in Hydrography for France, is amusing : ' Le passage 
de Venus,' he says, 'qui sans doute n'etait pas predit avec une pre- 



310 MYSTERIES OF TIME AND SPACE. 

Just as the transit of December 1874 was followed 
by another December transit in the year 1882, so the transit 
of December 1631 was followed by another in December 
1639. The earlier had escaped observation, as we have 
seen, though predicted by Kepler ; the latter, which accord- 
ing to Kepler's tables would not take place, was ob- 
served, though it almost escaped the ingenious astronomer 
who detected the mistakes in Kepler's computations and 
watched for its occurrence. This astronomer was one 
whose name is only not associated with any great discoveries 
because he died so young. Had he lived it is probable 
that Newton himself would not have stood much higher 
among the astronomers of England. Jeremiah Horrocks,. 
minister of Hoole, in Lancashire (aged only twenty), had in 
his zeal for science gone over the computations published 
by Kepler in the Rudolphine Tables. Comparing these 
with Lansberg's Tables of the Motions of Venus, he noticed 
that, while according to Kepler the planet would pass very 
close to the sun, but south of his disc, on December 4, 1639, 
Lansberg's Tables assigned to the planet at that conjunction 
a course traversing the northern part of the sun's disc. He 
had found Kepler a much more reliable authority than 
Lansberg ; but he had reason, from his own observations, 
to believe that Venus would follow a course between the 
two paths thus assigned by Lansberg and Kepler — somewhat 
nearer to Kepler's — insomuch that, instead of passing south 
of the sun, she would transit the southern part of his disc. 
He determined, therefore, to watch carefully for this 
interesting phenomenon. 'Lest a vain exultation should 
deceive me/ he says, ' and to prevent the chance of disap- 
pointment, I not only determined diligently to watch the 

cision suffisante, ne fut pas observe, cPabord parce que Gassendi, qui 
s'appretait a l'observation, en fut empeche par la pluie, mais surtotit 
parce que le passage eut lieu pendant la nuit pour les observateurs 
Europeens.' It may reasonably be admitted that the occurrence of the 
transit when the sun was below the horizon was a sufficient cause for 
Gassendi's failure, apart from the rain. 



TRANSITS OF VENUS. 311 

important spectacle myself, but exhorted others whom I 
knew to be fond of astronomy to follow my example, in 
order that the testimony of several persons, if it should so 
happen, might the more effectually promote the attainment 
of truth, and because by observing in different places our 
purpose would be. less likely to be defeated by the acci- 
dental interposition of clouds or any fortuitous impediment.' 
In fact, he was not free from astrological fears pointing to 
such interposition, since the positions of the planets Jupiter 
and Mercury seemed to portend bad weather. ' For/ he 
remarks, ' in such apprehension I coincide with the opinion 
of the astrologers, because it is confirmed by experience ; 
but, in other respects, I cannot help despising their puerile 
vanities.' 

Horrocks's description of his successful observation is 
interesting in many respects, especially perhaps for the en- 
thusiasm which pervades it. ' On voit,' says Delambre in his 
' History of Modern Astronomy,' ' que Horrocks etait jeune 
et enthousiaste, mais cette jeunesse et cet enthousiasme an- 
noncaient un horame vraiment distingue.' 'Following the 
example of Gassendi,' Horrocks begins, ' I have drawn up 
an account of this extraordinary sight, trusting that it will 
not prove less pleasing to astronomers to contemplate Venus 
than Mercury, though she be wrapt in the close embraces of 
the sun — 

Vinclisque nova ratione paratis 
Admisisse Deos. 

Hail ! then, ye eyes that penetrate the inmost recesses of 
the heavens, and gazing upon the bosom of the sun with 
your sight-assisting tube, have dared to point out the spots 
on that eternal luminary ! And thou, too, illustrious Gas- 
sendi, above all others, hail ! thou who, first and only, didst 
behold Hermes' changeful orb in hidden congress with the 
sun. Well hast thou restored the fallen credit of our ances- 
tors, and triumphed o'er the inconstant Wanderer. Behold 
thyself, thrice celebrated man ! associated with me, if I may 



312 MYSTERIES OF TIME AND SPACE. 

venture so to speak, in a like good fortune. Contemplate, 
I repeat, this most extraordinary phenomenon, never in our 
time to be seen again ! the planet Venus, drawn from her 
seclusion, modestly delineating on the sun, without disguise, 
her real magnitude, whilst her disc, at other times so lovely, 
is here obscured in melancholy gloom ; in short, constrained 
to reveal to us those important truths which Mercury on 
a former occasion confided to thee. How admirably are 
the destinies appointed ! How wisely have the decrees of 
Providence ordered the several purposes of their creation ! 
Thou ! a profound divine, hast honoured the patron of wis- 
dom and learning ; whilst I, whose youthful days are scarce 
complete, have chosen for my theme the Queen of Love, 
veiled by the shade of Phoebus' light' 

Horrocks was fortunate in possessing a telescope of con- 
siderable power (for that period). He says that it showed 
even the smallest spots upon the sun, and enabled him to 
make the most accurate division of the solar disc. More- 
over, he was already, notwithstanding his youth and the 
recentness of the invention, familiar with the use of the 
telescope, and he remarks respecting the instrument he 
employed that in all his observations he had found it repre- 
sent objects with the greatest truth. 

Horrocks' s calculations must have been made with great 
care and excellent judgment. We have seen that Gassendi's 
attempt to observe the transit of 1631 had failed, because 
Kepler's computations had been so far erroneous that, 
instead of the transit beginning before sunset on December 
6, it really began nearly at midnight (for Paris or Green- 
wich) between December 6 and December 7. Halley's com- 
putation of the transit of 1761 was also seriously in error 
(about half an hour) ; and an error of fully an hour was made 
in the first published statement respecting the transit of the 
year 1874 ! We shall see that Horrocks was correct within 
a few minutes. He had found that other astronomers set the 
conjunction of 1639 as occurring on November 23 (old style), 
whereas his own calculations ' forbad him to expect anything 



TRANSITS OF VENUS. 313 

before three o'clock in the afternoon of the 24th ' (Decem- 
ber 4, new style.) Fearing, however, lest ' by too much self- 
confidence ' he might ' endanger the observation,' he watched 
the sun through the greater part of the 23rd, and the whole 
•of the 24th. 'I watched carefully,' he says, 'on the 24th 
from sunrise to nine o'clock, and from a little before ten 
until noon, and at one in the afternoon, — being called away 
in the intervals by business of the highest importance, which, 
for these ornamental pursuits, I could not with propriety 
neglect.' As the day was Sunday, we may gather from this 
that the church services in small places like Hoole, in the 
seventeenth century, lasted not much longer than the low 
mass of a century earlier ; but that a longer service took place 
.at one, unless (as seems not unreasonable) we are to suppose 
that Horrocks took his dinner between one and three, soon 
.after which hour he resumed his observation of the sun. 
' About fifteen minutes past three,' he says, ' when I was 
again at liberty to continue my labours, the clouds, as if 
by Divine interposition, were entirely dispersed, and I was 
once more invited to the grateful task of repeating my ob- 
servations. I then beheld a most agreeable spectacle — the 
object of my sanguine wishes — a spot of unusual magnitude, 
and of a perfectly circular shape, which had already fully 
•entered upon the sun's disc on the left, so that the limbs of 
the sun and Venus perfectly coincided. Not doubting that 
this was really the shadow of the planet, I immediately ap- 
plied myself sedulously to observe it.' 

We need not consider very closely what Horrocks 
actually observed ; for of course no special scientific interest 
.attaches to the details of his observations. The chief point 
is that his prediction should have been so closely fulfilled. 
The time, indeed, during which he could examine the ap- 
pearance and motions of Venus was very short. He had 
first seen her at a quarter past three, and the sun set thirty- 
five minutes later. Nevertheless, he effected one discovery 
worthy of notice. He found that the planet's apparent size 
as very much smaller than had been supposed. Gassendi 



3H MYSTERIES OF TIME AND SPACE. 

had effected a similar discovery respecting Mercury. Thus 
had the transits of these two planets shown that relatively 
to the sun their globes are much smaller than astronomers 
had imagined. 

Horrocks had written to his friend Crabtree, a young 
man well skilled in mathematics and astronomy, ' inviting 
him to be present at this Uranian banquet, if the weather 
permitted.' ' But the sky,' says Horrocks, ' was very un- 
favourable, being obscured during the greater part of the 
day with thick clouds ; and as he was unable to obtain a 
view of the sun, he despaired of making an observation, and 
resolved to take no further trouble in the matter. But a 
little before sunset — namely, about thirty-five minutes past 
three — the sun bursting forth from behind the clouds, he at 
once began to observe, and was gratified by beholding the 
pleasing spectacle of Venus upon the sun's disc. Rapt in 
contemplation he stood for some time motionless, scarcely 
trusting his own senses, through excess of joy; for we astro- 
nomers have, as it were, a womanish disposition, and are 
overjoyed with trifles, and such small matters as scarcely 
make an impression upon others ; a susceptibility which 
those who will may deride with impunity, even in my own 
presence ; and if it gratify them I too will join in the merri- 
ment. One thing I request : let no severe Cato be seriously 
offended with our follies ; for, to speak poetically, what 
young man on earth would not, like ourselves, fondly admire 
Venus in conjunction with the sun, pdchritudinem divitiis 
conjunctam ? ' l 

None but these twa saw the transit of 1639. It might 
have been observed under more advantageous conditions by 
astronomers in Spain. In France it could have been seen 

1 This passage, like the others quoted respecting the transit of 
1639, is from the translation of Horrocks's original memoir, by the 
Rev. Arundell B. Whatton. The sketch of Horrocks's life, accom- 
panying Mr. Whatton' s translation, is full of interest. We are glad to 
see that the Astronomical Society has honoured itself recently by having; 
a tablet placed in Westminster Abbey in memory of Horrocks. 



TRANSITS OF VENUS. 315 

nearly as favourably as in England ; but in the eastern parts 
of Europe it could not have been seen at all. It would 
have been favourably seen from the greater part of the North 
American Continent, had there been any astronomers there 
to study it, 

Many years passed before the astronomical world began 
again to consider the subject of a transit of Venus. It was 
not, indeed, until June 1761 that another was to take place. 
Towards the close, however, of the seventeenth century, the- 
astronomer Halley, who during his stay (when very young) 
at St. Helena had observed a transit of Mercury, published 
an interesting dissertation showing how a transit of Venus 
might be so observed as to afford means for determining the 
distance of the sun. In this paper he discussed the circum- 
stances of the transit of 176 1, according to his calculation 
respecting the time and manner of its occurrence. It was 
then that he described the method of observing a transit, 
which is commonly called Halley's, though it may also be 
cahed (if we do not wish to give Halley his due) the method 
of durations. Probably the simplest sketch of this method 
would be thought out of place in these pages. We shall 
therefore content ourselves with merely noting that it de- 
pends on observing the duration of the transit as seen at 
different stations. If our earth were a mere point compared 
with the sun, the circumstances of a transit would be 
appreciably the same from whatever part of the earth it 
was observed. But as the earth has dimensions which, 
though small, are yet measurable compared with the sun's, 
observers in different parts of the earth see a transit under 
different circumstances; and amongst other circumstances 
affected in this way, the duration of the transit may be 
longer or shorter according to the observer's position. 
It does not matter whether the difference be brought 
about by setting one observer far to the north and another 
far to the south, or by taking advantage of the rotation of 
the earth which in a transit of long continuance shifts 
the. place of an observer who is near the equator very im- 



.3i6 MYSTERIES OF TIME AND SPACE. 

portantly, while scarcely at all affecting an observer placed 
in a high latitude, or even shifting him in a contrary direc- 
tion if he is on that side of the arctic regions which lies 
farthest from the sun. It is manifest that the larger the earth 
compared with the sun's distance, the greater will be the 
effect due to difference of position ; in other words, the 
difference of duration will be greater. And nothing can be 
simpler than the measurement of the transit's duration as 
observed at any place. All that is necessary is a clock 
which will not gain or lose appreciably during the time that 
the transit is in progress. Having determined the difference 
for stations of known position on the earth, we are enabled to 
infer what proportion the earth's dimensions bear to the sun's 
distance, or, in other words, we learn how far off the sun is. 

Halley, then, in the dissertation to which we refer, pro- 
posed that, during the transit of 1761 observers should be 
placed at certain stations which he pointed out — at Ben- 
coolen in Sumatra, at an arctic station near Hudson's Bay, 
and so on — where the transit would have its greatest and its 
least duration, so that by finding how great the difference 
of duration might be, the observers would be enabled to 
infer the relation which the earth's dimensions bear to the 
distance of the sun. 

The remarks with which Halley closed the introductory 
portion of his dissertation are worth quoting for the fine 
scientific spirit which pervades them : — ' I could wish,' he says, 
■ that many observations of this famous phenomenon might 
be taken by different persons at separate places, both that 
we might arrive at a greater degree of certainty by their agree- 
ment, and also lest any single observer should be deprived, 
by the intervention of clouds, of a sight which I know not 
-whether any man living in this or the next age will ever see 
again, and on which depends the certain and adequate solu- 
tion of a problem the most noble, and at any other times not 
to be attained to. I recommend it therefore again and again 
to those curious astronomers who, when I am dead, will have 
:an opportunity of observing these things, that they would 



TRANSITS OF VENUS. 317 

remember this my admonition, and diligently apply them- 
selves with all their might in making this observation ; and 1 
earnestly wish them all imaginable success ; in the first place 
that they may not, by the unseasonable obscurity of a cloudy 
sky, be deprived of this most desirable sight, and then, that 
having ascertained with more exactness the magnitudes of 
the planetary orbits, it may redound to their immortal fame 
and glory.' 

As the transit of 1761 drew near, careful observations of 
the motion of Venus were made, which showed that she 
would not transit the sun in the manner predicted by Halley. 
It was also found that no stations could be reached at which 
Halley's method could be conveniently applied. In fact 
the transit was not one whose whole duration could be 
advantageously observed. The French astronomer Delisle 
proposed another method, more difficult in an astronomical 
sense, but geographically much more convenient. We have 
already said that, owing to the fact that the earth has dimen- 
sions measurably comparable even with those of the solar 
system, the circumstances of a transit will be different at 
different stations. Amongst other circumstances so affected 
will be the time at which transit will begin or end. Halley's 
method was directed to the observation of the time transit 
lasts, and the differences observed in such duration were to 
give the means of determining the sun's distance. Delisle 
suggested that observers might note the time at which transit 
began or ended (not necessarily observing the whole transit), , 
and that differences observed in the epochs noted would 
give the means of measuring the sun's distance. Of course 
all the observed epochs would have to be expressed accord- 
ing to a uniform manner (all in Greenwich time, for instance, 
or all in Paris time) ; so that the longitude of each observing 
station would have to be determined as well as the local 
time at which transit began or ended. But this accom- 
plished, the method would avail as well as Halley's for 
determining the sun's distance. 

Accordingly, the French Academy made preparations for 



318 MYSTERIES OF TIME AND SPACE. 

sending out observers to stations suitable for applying this 
method, which (if we wish to avoid naming Delisle), may be 
called th£ ■ absolute time method/ because it depends on 
comparing the absolute instants at which transit begins or 
-ends at different stations. Le Gentil, most unfortunate of 
men so far as transits of Venus were concerned, was sent to 
Pondicherry, but, as we shall presently see, did not arrive 
there. Here the whole duration could be observed, but 
his special object was to observe ingress. Chappe d'Aute- 
roche was sent to Tobolsk in Siberia. England, with fine 
official pertinacity, held fast to Halley's selection, Bencoolen, 
though in the actual circumstances of the transit Bencoolen 
presented no advantages whatever. Doubtless an expedition 
would have been sent to Halley's other station near Hud- 
son's Bay, but for the circumstance that the transit would 
not have been visible there at all. 

The actual history of the expeditions is full of interest, 
but would require more space than can here be devoted to 
it. Fortunately for science, the ship which had set out 
for Bencoolen was attacked by a Spanish vessel of greatly 
superior strength, and being compelled to put in at the Cape 
of Good Hope, an excellent station for observing egress in 
Delisle's manner, observations were made which proved far 
more serviceable than any which could possibly have been 
made at Bencoolen. Le Gentil set forth for Pondicherry 
on March 26, 1760. Had he reached that station, he would 
have witnessed the curious spectacle of a transit, the middle 
of which occurred with the sun almost vertically overhead. 
But when Le Gentil arrived at the Isle of France (on July 
10, 1760), he learned that a war had broken out between 
France and England, and that it would be unsafe for his 
ship to proceed. He had resolved to betake himself to 
Rodriguez (which would, however, have been a most unfor- 
tunate selection, as only the egress would have been visible, 
and under very unfavourable conditions), when he learned 
that a French frigate was about to sail for the coast of Coro- 
mandel. In her, therefore, he determined to proceed to 



TRANSITS OF VENUS. 



.19 



Pondicherry. He sailed from the Isle of France in the 
middle of March 1761, and after experiencing many provok- 
ing delays from calms, reached Malabar on May 24, only to 
learn that the English were masters of Pondicherry. The 
captain of the frigate sailed away with all speed for the Isle 
of France, and she was still on her way thither when the day 
of the transit arrived. Le Gentil made most ingenious pre- 
parations to observe the transit, and favoured by splendid 
weather he had an excellent view of its phenomena. But it 
need hardly be said that the observations he made, however 
interesting to him as a student of astronomy, were utterly 
valueless for the determination of the sun's distance. As 
Dubois well remarks, • Le Gentil had experienced one of 
those mishaps which assume to the man of science all the 
proportions of a real misfortune ; to have traversed so large 
a portion of the globe, to have endured all the weariness, 
all the privations, all the perils, of a long sea- voyage, and to 
effect nothing, this was enough to have disgusted anyone 
with scientific observation, or, at least, with Halley's method.' 
But, as we shall soon see, Le Gentil was even now not at 
the end of his troubles. Chappe d'Auteroche, after a long 
and painful journey, reached Tobolsk, and observed the 
transit there under favourable conditions. 

The results of the observations of the transit of 1761 
were by no means so satisfactory as had been expected, so far 
as the determination of the sun's distance was concerned. 
Some estimates made the distance nearly 100 millions of 
miles, while, according to others, the sun's distance fell short 
of 80 millions of miles. Astronomers had already obtained 
much more satisfactory measurements from observations 
made upon the planet Mars; so that the most striking result 
of the transit observations made in 1761, seemed to be the 
recognition of the inferiority of such observations compared 
with other methods available to astronomers for determin- 
ing the sun's distance. 

It was, in fact, during the transit observations of 176 1 
that astronomers recognised a peculiarity in the behaviour 



320 MYSTERIES OF TIME AND SPACE. 

of Venus as she enters upon and leaves the sun's disc, which 
militates very strongly against the usefulness of both Halley's 
and Delisle's method. Theoretically nothing can be more 
perfect than the plan suggested for timing durations by one 
method, and the absolute moment of ingress or egress by 
the other. As the round black disc of Venus passes upon 
the sun's face at ingress, the observer can wait for the 
moment when her outline will just touch the sun's. As this- 
moment gradually approaches, he can give his whole atten- 
tion to determine the exact instant when the two outlines 
are in contact ; and theoretically this instant can be exactly 
determined : for, the moment after, the sun's light will be 
seen between the two outlines. But practically matters do 
not proceed so conveniently. In the first place, the outline 
of orb is rippled through the effects of atmospheric undula- 
tions, and so much the more disturbed as the sun is nearer 
the horizon, — a fact of great importance, because all the 
best stations have the sun somewhat low down at the critical 
moment of contact. Then there is the optical effect called 
irradiation, by which the image of a bright object on the 
retina is apparently enlarged. This effect, of course, makes 
the dark disc of Venus look smaller, and the bright disc of 
the sun look larger than they really are ; and a very little 
consideration will show that at the moment when the two 
are really in contact (Venus just lying wholly within the sun's 
disc) instead of appearing as a round black disc just touch- 
ing the sun, she will appear to have a disc smaller than her 
real orb, and not round, since irradiation will not contract 
the disc at the place of contact as it does elsewhere. Thus 
at the moment of real contact Venus will present a pear- 
shaped aspect, the stalk of the pear connecting the body 
with the edge of the sun's disc. The moment after real 
contact, the stalk will appear to break, and either in an 
instant or very quickly the pear-shape will disappear, and 
the disc will become circular, with a wide space separating 
it from the edge of the sun's disc. The disc will seem to 
have taken a leap, as it were, from the sun's edge, to which 



TRANSITS OF VENUS. 321 

a moment before it was attached by the black stalk. Similar 
phenomena will present themselves at egress in a reverse 
order. Now, these effects are combined with those due to 
atmospheric undulations • and moreover the extent of irra- 
diation depends largely on the observer himself (some per- 
sons being much more sensitive to the effects of light than 
others), and largely also on the telescope employed, the state 
of the air, and other variable circumstances. So that mani- 
festly, instead of that neat and precise determination of the 
moment of contact which Halley expected, and which both 
his method and Delisle's require, there must be considerable 
uncertainty in the comparison of observations made by 
different astronomers, at different stations, with different 
telescopes, and under different conditions. 

Most of those who observed the transit of 176 1 mention 
the occurrence of peculiarities such as we have here de- 
scribed. Thus, Mr. Hirst, who observed the transit at 
Madras, where the sun was at a considerable elevation, 
states that 'at the total immersion, the planet, instead of 
appearing truly circular, resembled more the form of a 
Bergamot pear, or, as Governor Pigott then expressed it, 
'looked like a nine-pin/ yet the part of the disc farthest 
from the sun was extremely well-defined. When the planet 
was about to leave the sun similar appearances were pre- 
sented. ' The planet was as black as ink, and the body- 
truly circular just before the beginning of egress, yet it was 
no sooner in contact ' with the edge of the sun's disc ' than 
it assumed the same figure as before,' the other part of 
Venus 'keeping well-defined and truly circular.' 

However, the general impression among the astronomers 
of the last century would seem to have been that too much 
reliance had been placed on Delisle's method. Cassini, 
several years later, wrote as follows respecting the arrange* 
ments for the transit of 1769 : — 'Experience is our chief 
instructor ; the fruit of her lessons repays us for the years 
passed in learning them. In 1761 the principal object had 
been missed for want of observations in places where the 

Y 



322 MYSTERIES OF TIME AND SPACE. 

durations differed sufficiently. It was essential not to 
experience a second time the same disadvantage.' 

In accordance with this view, preparations were made 
for sending observers in 1769 to the South Sea, California, 
Mexico, Lapland, Kamschatka, Hudson's Bay, and other 
places where the whole transit could be observed. England 
took an important part in these arrangements. At that period 
Spain possessed nominal dominion over the South Sea, and 
the French .Government waited for permission from Spain 
to observe the transit in those seas. This permission Spain 
refused to grant. But England did not wait for it. Indeed, 
from all the accounts we have seen, it does not appear that 
the idea of asking Spain for permission to visit the Southern 
Seas occurred to the British authorities. The Royal Society 
presented a memorial to George III. early in 1768, request- 
ing, among other things, that a vessel might be fitted out at 
the expense of Government ' to convey proper persons to 
observe the transit, either from the Marquesas, or from one 
of those islands to which Tasman had given the several 
appellations of Amsterdam, Rotterdam, and Middleburgh," 
now known as the Friendly Isles. This petition was readily 
complied with. But early in the negotiations a hitch oc- 
curred which threatened mischievous delays. Dalrymple, 
who had been selected to superintend the expedition, was 
a man eminent in science, and every way worthy of con- 
fidence ; moreover he had ' already greatly distinguished 
himself respecting the geography of the Southern Ocean. 
As this gentleman had been regularly bred to the sea, he 
insisted on having a brevet commission, as captain of the 
vessel, before he would undertake the employment.' The 
Admiralty violently opposed this measure, and Lord Hawke 
(who then presided at the Admiralty) ' declared that nothing 
could induce him to sanction such a commission.' After 
much debate, both sides proving inflexible, it was thought 
desirable to look out for another commander for the 
expedition, and Captain Cook (then Lieutenant and after- 
wards world-renowned) was eventually appointed, The 



TRANSITS OF VENUS. 323 

c Endeavour,' a barque of 370 tons, originally built for the 
coal trade, was selected as a suitable vessel, and at the 
suggestion of Captain Wallis, who had just returned from a 
voyage round the world, Otaheite (then called King •George's 
Island) was chosen as the most convenient place for ob- 
serving the transit. The observers selected were Mr. Charles 
Green, assistant of Dr. Bradley, the Astronomer Royal, and 
Mr. Joseph Banks (afterwards the President of the Royal 
Society). Two draughtsmen, a secretary, and four subordi- 
nate assistants accompanied the observers. Solander, the 
Swedish naturalist, also sailed with Cook, and his botanical 
observations were among the most important fruits of the 
expedition. The transit was successfully observed both by 
Green and Banks. 

Le Gentil experienced in 1769 the culmination of his 
misfortunes as a transit observer. With a pertinacious 
courage worth of better success, he determined, after his 
failure in 1761, to return to Pondicherry so soon as an 
opportunity- presented itself, and there to await the transit 
of 1769. For eight years he waited, employing himself in 
the agreeable study of Brahminical astronomy. But, alas ! 
when June 3, 1769, arrived; an envious cloud covered the 
sun at the moment when Le Gentil was preparing to reap 
the reward of his patience ; and all that was left to the 
unhappy astronomer was to return to France and publish a 
book on the astronomy of the Brahmins. Let us hope that 
he was kindly treated by the critics. 

As important as observations in the South Seas, where 
the duration of the transit was shortened, were those made 
at Wardhuus, in Lapland, where the duration was length- 
ened. The King of Denmark invited Father Hell, a skilful 
German astronomer, to occupy this station in company with 
the Danish astronomer Borgrewing, Arriving at Wardhuus 
in the autumn of 1768, the two astronomers wintered in 
that desolate region ; and fortunately, when the day of the 
transit arrived, clear weather permitted them to make good 
observations. Doubt has, indeed, been thrown upon the 

y 2 



324 MYSTERIES OF TIME AND SPACE. 

observations of Hell, in comparatively recent times, because 
of the difficulty of reconciling them with the present estimates 
of the sun's distance. The Astronomer Royal has even gone 
so far as to suggest that the worthy Father was asleep at the 
moment of egress, and that being ashamed to admit the fact 
he made an entry in his notebook describing an imaginary 
observation, We know of nothing rendering this at all 
probable, for Hell was a man held in high esteem by his 
contemporaries. The time, indeed, at which egress oc- 
curred was such that sleep would not in itself have been 
an improper indulgence. Transit began, at Wardhuus, at 
about half-past nine in the evening, and ended at about 
half-past three in the morning (there was no night in that 
high latitude), and a rest in the interval would have been 
excusable. But it seems absurd to suppose that the astro- 
nomer would have left his waking to chance. Besides, 
there were the astronomers Sajnowiz and Borgrewing, as 
well as several assistant observers, and these would not 
have left Father Hell to sleep through the important 
moments of egress. It seems likely, in fact, that his 
unlucky name, rather than any other circumstance, suggested 
a charge which, if really warranted, would expose him to 
the undying obloquy of astronomers. In this respect he 
may be compared to that unfortunate Dr. Impey, whom 
Macaulay represents to us in so contemptible an aspect, — 
with no better justification, according to well-informed 
historians. 

A great deal has been said during the last few years 
about the error which astronomers are supposed to have 
detected in that estimate of the sun's distance which had 
been based on the observations of 1769. But in point of 
fact it was very early seen that these observations were little 
more trustworthy than those made in 1761. Within less 
than two years, upwards of two hundred papers containing 
different estimates of the sun's distance were sent by various 
persons to the Academy of Paris, and probably about four 
hundred to the different learned societies of Europe. 



TRANSITS OF VENUS. 325 

Selecting from among these the papers contributed by the 
able mathematicians, Lalande, Euler, Pingre, Hornsby, and 
Hell, we find the estimates of the sun's distance ranging 
from 92 millions to 96 millions of miles. Not the least 
curious part of the matter is that all the calculators were 
positive they were right. Pingre, who made the distance 
92 millions of miles said, ' Of two things one, either this is 
the true distance, or the observations of 1769 are not to be 
trusted at all ; ' while Lalande said that ' incontestably ' the 
sun's distance amounts to fully 96 millions of miles. Euler, 
a greater mathematician than either, after carefully going 
over his work afresh, obtained a value almost exactly midway 
between Lalande's and Pingre's. Then did Dionis du 
Sejour publish a new investigation leading to nearly the 
same value which Pingre had obtained. Lastly came 
Encke, who with German patience combined all the ob- 
servations together, with a result nearly coinciding with 
Lalande's. This was the value of the sun's distance — 
95,265,000— which for so many years reigned in our books 
of astronomy ; though why implicit reliance was given to 
it when the history of the investigation showed that mathe- 
maticians more skilful than Encke had obtained results 
differing widely from his, cannot be easily explained. Nor 
should it have been thought at all a wonderful circumstance 
that researches by other methods soon began to point to a 
different value of the sun's distance. 

This would not be the place to explain the various 
methods by which, without the aid of a transit of Venus, 
astronomers have in recent times obtained new estimates of 
the scale on which the solar system is constructed. Yet the 
ingenuity with which the great problem has been attacked 
is so remarkable that it may interest our readers to have 
simply stated, without explanation of details, the con- 
trivances employed by astronomers. First, there was an old 
method depending on the observation of the planet Mars, 
when at his nearest to us, and when therefore his apparent 
position in the heavens is most affected by the difference in 



326 MYSTERIES OF TIME AND SPACE. 

the position of the observers on our earth. This method 
was applied in two ways : in one, by stationing observers 
far apart ; in the other, by taking advantage of the fact that 
an observer is carried round by the daily rotation of the 
earth so as to have his place changed for him, so to speak. 
Then the motions of the moon were consulted. Our 
satellite is disturbed by the sun, and if her path were 
indefinitely small compared with his distance she would 
be just as much disturbed in the half of her path farthest 
from him, as in the half nearest to him ; but as the sun's 
distance is not immeasurably superior to the moon's, she 
is slightly more disturbed when traversing the half of her 
path nearest to him than when traversing the other half. 
The excess of disturbance being noted, affords a means 
of estimating the sun's distance ; for, as we have seen, it 
depends on the extent to which that distance exceeds the 
readily measured distance of the moon. 1 Then there was 
yet another method, depending on the fact that the earth 
circuits once a month around the common centre of gravity 
of her orb and the moon's, so that she is now a little on 
this side, now a little on that side, of the place she would 
have if there were no moon. This slight range on her part 
from what may be called her mean position gives, as it were, 
a base of measurement, to either extremity of which the 
astronomer is carried successively month after month, and 
the resulting slight displacement of the sun's apparent place 
shows itself in the records of Greenwich, Washington, Paris, 
and other great observatories. The sun's distance is inferred 
from such observations, just as a surveyor infers the distance 
of some inaccessible spire or rock by noting how much it is 
shifted in direction as seen from one or the other end of a 
measured base-line. Then there was that most ingenious 
and wonderful of all methods which depends on the mea- 

1 The moon's distance is easily measured because she is so near 
that two observers at distant stations on the earth see her in perceptibly 
different directions. 



TRANSITS OF VENUS. 327 

surement of the velocity of light. Everyone knows that 
astronomy first revealed the fact that light travels with a 
measurable though inconceivable velocity, The little satel- 
lites of Jupiter were found to undergo eclipse earlier or later 
according as Jupiter was nearer to or farther from us, and it 
was soon seen that these effects arise from the fact that the 
light-message by which the news of these eclipses is con- 
veyed takes a longer time to traverse the longer distance, — 
in other words, that light does not travel with infinite 
velocity. It appeared from the observed effects that light 
occupied about seventeen minutes in traversing a distance 
equal to the diameter of the earth's orbit ; and using 
Encke's value of the sun's distances, this implied that light 
travels with a velocity of about 192,000 miles per second. 
Of course if the sun's distance is greater, light travels more 
quickly, and if less then less quickly. It occurred to 
Foucault to apply an ingenious contrivance, devised by 
Wheatstone for measuring the duration of the electric 
spark, to the less difficult task of measuring the velocity of 
light. And inconceivable though it may seem that a velocity 
of nearly two hundred thousand miles per second can be 
measured by any terrestrial contrivance, the task was ac- 
complished so satisfactorily that the resulting estimate of 
the velocity of light has been thought a sufficient ground for 
adopting a new estimate of the sun's distance. Foucault 
found that light does not travel at so great a rate as 
192,000 miles per second, but at the rate of about 180,000 
miles, so that the diameter of the sun's orbit must be less 
than Encke had supposed, — the sun's distance being reduced 
in this way from 95,265,000 miles to about 92 millions. The 
values obtained by the other methods all lie much nearer 
to this value than to Encke's, ranging in fact from 91,230,000 
miles to 92,680,000 miles. So that whether Pingre and 
Dionis du Sejour were right or wrong in asserting that.the 
transit observations in 1769 point to a solar distance of 
92,000,000 miles, it is certain that modern observations 
point to such a distance. 



328 MYSTERIES OF TIME AND SPACE. 

Much has been said respecting the efforts which have 
been recently made to show that the observations of 1769 
can be forced into agreement with the new and reduced 
estimates of the sun's distance. The continental astronomer 
Powalky effected this by selecting certain observations and 
rejecting others — without giving any sufficient reasons for 
so doing. Stone, of Greenwich, adopted a plan little more 
satisfactory, though many writers (the present writer among 
the number) somewhat hastily assumed that he had re- 
moved the whole difficulty. We have described the peculiarity 
which affects the appearance of Venus when she is just 
wholly upon the disc of the sun. Between the moment at 
ingress when her rounded outline seems to belong to a 
circle which (if complete) would touch the sun's outline 
(the moment of apparent contact) and the moment when 
she seems suddenly to break away from the edge of the sun 
(the moment of real contact), an interval elapses ; and there 
is a corresponding interval between the two contacts at 
egress. Mr. Stone found that if this interval be taken as 
seventeen seconds then the observations of 1769 point to 
just such a distance of the sun as astronomers have recently 
been led to adopt. This is all very well ; but if it proved 
anything it would prove that the interval either always 
amounts to seventeen seconds or that seventeen seconds is 
a fair average value. Even if this were true nothing else 
would have been demonstrated by Mr. Stone's investigation. 
But unfortunately those observers who, availing themselves 
of the experience obtained in 1761, were careful to observe 
both kinds of contact in 1769, found the interval to be not 
only widely variable but always much greater than seventeen 
seconds. Green at Otaheite found the interval to be 40 
seconds at ingress and 48 seconds at egress. Cook made it 
60 seconds at ingress and 32 seconds at egress. Maskelyne, 
the Astronomer Royal, observed a difference of 52 seconds, 
while Horsley, at the same station (Greenwich Observatory), 
found it to be 63 seconds. Hornsby at Oxford found 
the difference to be 57 seconds, while Shuckberg, also at 



TRANSITS OF VENUS. 329 

Oxford, found it to be fully 69 seconds. Yet all these 
observers were prepared for this peculiarity, and Maskelyne 
had issued special instructions for their guidance in this 
particular respect. 1 We cannot wonder, therefore, if conti- 
nental and American astronomers unanimously decline to 
recognise any independent value in Mr. Stone's attempted 
reconciliation between the transit observations of 1769 and 
recent measurements of the sun's distance. 

It was hoped, however, that the observations which 
were to be made during the transits of 1874 and 1882 
would remove all doubt as to the correctness of these more 
recent measurements. At a very early date attention was 
directed to the transits by the Astronomer Royal for Eng- 
land ; who, in May 1857, delivered an address to the 
Astronomical Society, in which he described the various 
methods available for determining the sun's distance, and 
pointed out the advantages of a transit of Venus, more 
especially if it could be observed by Halley's method. He 
stated, however, that the method of durations could only 
be applied in 1882, any observable difference in 1874 ' being 
probably little more than half as great as in 1882.' It 
appeared also from his calculations that to apply the method 
successfully in 1882, Antarctic stations must be reached. 
Not deterred, however, by this difficulty, and remembering 
doubtless that British seamen were not altogether without 
fame as Antarctic explorers, he boldly advocated the occu- 
pation of Antarctic stations. The whole region, he said, 
' should be reconnoitred some years before the transit,' for 
' the future astronomical public will not be satisfied unless 
all practicable use be made of the transits of Venus in 1874 
and 1882.' 

In 1864 these suggestions were renewed, special atten- 
tion being directed to Sabrina Land and Repulse Bay. In 
May 1865, the Astronomer Royal heard that the Geo- 

1 One observer, at Caen, using a very small telescope, found the in- 
terval between real and apparent contact to be more than two minutes 
and a half—' a monstrous cantle out,' 



330 MYSTERIES OF TIME AND SPACE. 

graphical Society was endeavouring to move Government 
to send an expedition towards the North Pole, and he im- 
mediately put in a plea for an Antarctic expedition. ' In 
the year 1882,' he said, 'a transit of Venus over the sun's 
disc will occur ; the most favourable of all phenomena for 
solution of the noble problem of determining the sun's 
distance from the earth.' He then stated that the southern 
stations must be on the Antarctic continent, and pointed 
out that although, if such an expedition were undertaken 
the astronomical observations must take precedence of all 
others, ' there would be no difficulty in combining with them 
any other inquiries of geography, geology, hydrography, 
magnetism, meteorology, natural history, or any other sub- 
ject for which the localities are suitable.' 

But it was in December 1868 that the suggestions for 
Antarctic reconnaisance first took definite form. Then did 
the A stronomer Royal marshal an array of naval authorities 
— Admiral Richards (Hydrographer to the Admiralty), 
Admiral Ommanney, Commander Davis (a companion of 
Sir James C. Rcss in his celebrated Antarctic expedition), 
Captain Toynbee, and others — in support of the schemes 
which during the preceding eleven years he had from time 
to time advocated ; and, with excellent unanimity, these 
authorities expressed their belief that Antarctic explorations 
could be usefully and safely carried out. 

Hitherto the astronomers of other countries had taken 
no part in these preliminary inquiries and suggestions. It 
was doubtless felt that the matter could be well left in the 
hands of so excellent a mathematician as Sir George Airy. 
We find, in fact, that in the communication addressed in 
1868 to the Astronomical Society, the part which other 
nations were to take was indicated as well as that which our 
country might regard as specially her own. The lion's share 
was taken indeed for England, which was to occupy in 
1874 all the four regions suitable for observing by Delisle's 
method, while in 1882, besides taking her share in applying 
this method, she was to occupy stations on the Antarctic 






TRANSITS OF VENUS. 331 

continent. It is rather singular that no part whatever was 
assigned to America until 1882, when ' the utmost reliance 
might be placed on the zeal of our American brethren ; for 
observing the transit of that year at stations in the United 
States. 

But the next few years saw all these ideas changed. 
It was found that an error had been made in the original 
investigation of the conditions of the two transits, and that 
it is for the later not the earlier transit that Halle/s method 
fails. Accordingly, ample preparations were made for ob- 
serving the duration of the transit of 1874 from suitably 
selected stations. Among these were several which had 
been already chosen for the other method ; but others 
were new. In particular the Russian Government provided 
for a whole range of stations in Eastern Siberia where the 
transit had a lengthened duration, while America, France, 
and Germany arranged to occupy Crozet Island, St. Paul's 
Island, Campbell, Auckland, and other islands in the sub- 
Antarctic Seas, which, with Rodriguez, Kerguelen, and 
other places to be occupied by England, formed ample 
provision for the observation of the shortened duration. 

A little disappointment was occasioned to those who 
may have hoped, from the plans published in 1868, that 
Antarctic exploration would have been undertaken for 
observing the transit of 1882. So soon as it was pointed 
out that no good could result from the occupation of 
Antarctic stations in that year, all those plans were very 
properly abandoned. It had now become known, however, 
that Antarctic stations would be more useful during the 
transit of the present year than it had been supposed they 
would be in 1882. Some imagined that the authorities who 
had been so enthusiastic in favour of Antarctic exploration 
for one transit would not be altogether opposed to such 
exploration for the other. This hope was doomed to be 
disappointed. In fact the Astronomer Royal and the 
Admirals grew quite facetious in ridiculing the idea of Ant- 
arctic exploration ; though they suddenly became serious^ 



332 MYSTERIES OF TIME AND SPACE. 

even to severity, when reminded of the views they had 
themselves expressed in 1868. Thenceforth they deprecated 
jesting with a touching solemnity. 

But after all, Antarctic exploration was not a point of 
great importance for the transit of 1874. So admirably was 
the method of durations suited for this transit, that without 
incurring the dangers of Antarctic voyaging — whether these 
dangers be excessive, as now stated by the Admiralty, or 
slight, as they stated in 1868 — a large number of stations 
could be occupied both in the northern and southern hemi- 
sphere, whence the whole transit could be seen. And for- 
tunately for science the opportunity was recognised early 
enough to be turned to good account. Russia, as we have 
stated, occupied no less than eleven northern stations for 
observing the whole transit, America, Germany, and France 
occupying between them seven or eight others in Siberia, 
North China, and Japan, while England occupied one in 
North India. 

In the southern hemisphere nearly all the stations were 
such that the duration of the transit could be observed, 
except Cape Town, which had special value as a station 
for observing the middle of the transit. England occu- 
pied four stations in the southern hemisphere, besides Cape 
Town, Melbourne, Sydney, and other places already pro- 
vided with astronomical instruments ; and America, France, 
and Germany occupied many other southern stations for 
applying Halley's method. But it is not by any means 
to be supposed that Delisle's method was neglected. Eng- 
land, for instance, occupied the Sandwich Islands where 
only the ingress could be observed, and the Isthmus of 
Suez whence only the egress could be observed, and Russia 
had a yet larger number of astronomers devoted to the 
observation of egress only. Moreover, all the stations 
whence the duration could be seen were excellent stations 
for observing ingress and egress alone, so that where bad 
weather unfortunately prevented the observers from noting 
the duration, they still had a chance of doing useful work. 



TRANSITS OF VENUS. 333 

This, in fact, was one great reason why it would have been 
little less than a disaster for science had the value of the 
transit for Halley's method not been noted in good time ; 
because it was hardly to be expected that other nations 
would occupy second-rate Delislean stations when England 
and Russia had all the best stations of that kind, whereas 
under the actual circumstances a large number of second- 
rate but excellent Delislean stations were occupied because 
they were first-rate Halleyan stations. 



334 MYSTERIES OF TIME AND SPACE. 



STAR-CLOUDS AND STAR-MIST. 

There are some scientific questions which have excited an 
interest seeming at a first view disproportioned to their 
intrinsic importance. Sometimes the distinguished position 
of those who have taken part in scientific discussion, at 
others the skill and acumen with which rival theories have 
been maintained, have directed exceptional attention to par- 
ticular questions. Too often personal animosities have 
become associated with a subject of discussion ; for un- 
fortunately it sometimes happens that 

The man of science himself is eager for glory and vain, 
An eye well practised in nature, a spirit bounded and poor. 

But there have also been occasions when the singular 
progress of a dispute, the swaying hither and thither of 
contending evidence, and even the repeated apparent settle- 
ment of the question, by what seems like overwhelming 
evidence on one side or the other, have given the discussion 
a singular (perhaps a factitious), and in a sense an almost 
romantic interest. 

Among questions of this sort, few have been more 
interesting than the subject of which I propose to give now 
a short sketch, partly historical, partly explanatory — the 
history running up to this present time, the explanation to 
be completed only hereafter, and perhaps at a very remote 
date, if at all. 

When the depths of the heavens are explored with a 
powerful telescope a number of strange cloud-like objects 



STAR-CLOUDS AND STAR-MIST. 335 

are brought into view. It is startling to consider that if the 
eye of man suddenly acquired the light-gathering power of 
a large telescope, and if at the same time all the single stars 
disappeared, we should see on the celestial vault a display 
of the mysterious objects called nebulae or star- clouds, 
exceeding in number all the stars which can now be seen 
on the darkest night in winter. The whole sky would seem 
mottled with these singular objects. With reference to nebulae, 
or rather to certain classes of them, opposite views were 
for a long time maintained by astronomers. 'Whether,' 
said Humboldt, half a century since, 'the cloudy masses 
referred to be indeed composed of luminous matter, or 
whether they are merely remote, closely crowded, and 
rounded clusters of stars, is a question which has for more 
than two hundred years been agitated among astronomers.' 
Galileo and Kepler and Halley discussed the question 
when as yet the nebulae might be counted on the fingers of 
one hand. I might, indeed, even go further back, and quote 
the views of Tycho Brahe, who wrote before the telescope 
was invented. Later, the Cassinis and Mitchel, on one side. 
Durham, Lacaille, and Kant, on the other, supported the 
rival theories according to the evidence available in their 
day. Then came the wonderful labours of Sir William 
Herschel, who in an age when nebulae had been counted by 
tens, continued to send in to the Royal Society lists of 
thousands of these objects. His discoveries attracted fresh 
attention to the subject of controversy. Holding first the 
opinion that all nebulae are star-clouds — that is, closely 
aggregated congeries of stars, reduced by distance to the 
appearance of clouds — Herschel came round in the latter 
part of his career to the contrary view, and gradually, after 
many disputes among his followers and opponents, the 
question was held to be settled in favour of the opinion he 
maintained. How, since then, a contrary opinion- gradually 
struggled into favour, and was eventually established, as was 
thought, upon the firmest possible basis ; how this opinion, 
at the moment when it seemed that all astronomers were 



336 MYSTERIES OF TIME AND SPACE. 

about to] accept it, suddenly and unexpectedly received 
the coup de grace from an eminent physicist of our day ; 
and how later the simple interpretation, which seemed thus 
suggested, has been in turn rendered doubtful by fresh 
evidence, is what I now propose to explain. 

It will not be uninteresting, however, to examine briefly 
in the first place some of the more striking features of the 
wonderful universe of nebulae. There are, indeed, few 
subjects in astronomy better calculated to excite emotions 
of astonishment and awe than those which are associated 
with the remotest depths of space yet reached by the tele- 
scope. The feeling with which men in ah ages regarded the 
star-lit vault of heaven is far less intense than that with which 
the astronomer gazes into depths ' to which the visible skies 
are but as the portal,' and analyses the fantastic star-clouds 
which come into view with each increase of telescope 
power. ' System on system of majesty unspeakable, float,' 
said the late Professor Nichol, ' through the fathomless 
ocean of space. Our galaxy, with splendours that seem 
illimitable, is only an unit among unnumbered throngs ; we 
can think of it, in comparison with creation, but as we were 
wont to think of one of its own stars.' But now a yet more 
startling view of the habitudes of space is presented 
to our contemplation. Recognising the possibility that 
some nebular objects which the telescope reveals to us may 
be systems of stars external to our own, and in a sense 
resembling it, we are yet forced to recognise the existence 
of vast wildernesses of matter presenting no characteristics 
such as we had become accustomed to. We learn to 
recognise in our own stellar system a far more varied 
structure than we had hitherto supposed it to have ; while 
as respects external galaxies, not new systems merely, but 
new kinds of systems are revealed, while the imagination is 
left to picture yet more wonderful systems of systems within 
depths to which the most powerful telescopes yet made by 
man cannot penetrate, 

It was early recognised by Sir William Herschel that the 



STAR-CLOUDS AND STAR-MIST. 337 

nebulae admit of being arranged into certain very distinct 
classes. There are the star-clusters, splendid gatherings of 
stars, the most striking perhaps of all telescopic objects. 
Some of these are so magnificent that what Professor Nichol 
said of them is probably not far from the truth, viz. : ' That 
no one has ever seen them in a telescope of adequate power 
without uttering a shout of wonder.' Then, as though by 
mere increase of distance, we have star-groups of less 
splendour, their constituent orbs more and more closely 
congregated, until at length we either reach depths at which 
it is no longer possible to distinguish separate stars, or else 
we examine regions where the stars are in reality so much 
smaller and so much more closely set that -they seem to 
form mere aggregations of star- mist. Among such telescopic 
objects are to be seen nebulae whose characteristics suffice 
to show that could we only increase the power of our tele- 
scopes we should discern the separate stars now lost to us 
under a milky haze. A sparkling appearance which the 
practised observer cannot fail to recognise indicates that the 
source of the light consists of a multitude of brilliant orbs, 
and not of a luminous haze or mist. 

Respecting the important classes of nebulae just spoken 
of, but one opinion prevailed till recently among astro- 
nomers. These clusters have been held to be so many 
distinct aggregations of stars scattered throughout the depths 
of space at various distances, many of them comprising within 
their bounds a far larger number of orbs than the naked eye 
perceives upon the darkest and clearest night. The same 
opinion was also formed of those objects which, in the mag- 
nificent telescope of Lord Rosse, have been found to present 
a spiral or whirlpool appearance. Indeed, there were those 
who looked upon our galaxy as being a member of the 
same class, so that to astronomers in those outlying uni- 
verses the Milky Way, and all the stars which deck our 
nocturnal skies, would seem reduced to a small spiral coil 
of hazy light. Whether this opinion, or the general view 
which associates the stellar cluster with a theory of external 

z 



338 MYSTERIES OF TIME AND SPACE. 

galaxies, be correct, it is not here my special purpose to 
inquire. I mention the view because it for a long time 
held undisputed sway among astronomers, and because it 
conveniently illustrates the aspects of certain classes of 
nebulae. 

What was disputed among astronomers, even in the 
time when the theory just described was accepted, was the 
question whether we can pass from the forms of nebulae 
above described to those which remain to be considered, or 
whether a broad line of demarcation separates the latter 
entirely from the former, and forces us to look upon them 
as objects of quite another kind. 

In searching over the heavens Sir William Herschel 
came across certain objects which might very well at first 
sight be mistaken for tailless comets, or even for enormous 
planets removed to so great a distance from the sun as to shine 
with a very feeble light. He called these objects planetary 
nebulae. As described by him they presented a most 
perplexing subject of inquiry. ■ They are/ he wrote, ' some- 
what extraordinary objects, with round or slightly oval disks, 
in some instances quite sharply terminated, in others a little 
hazy at the borders, and of a light exactly equable, or only 
a very little mottled, which in some of them approaches in 
vividness to the light of actual planets.' 

It was principally the examination of these strange 
objects that led Sir William Herschel to abandon the view 
that all nebulae are composed of stars. It was the peculi- 
arity of this admirable astronomer that he added to extra- 
ordinary skill as an observer an acumen in the interpretation 
of observations which has never perhaps been equalled. I 
may notice in passing that the two qualities are seldom 
found in combination, at least in their highest excellence. 
Observers of the utmost eminence might be named who 
have been wholly wanting in the power of drawing from 
their observations the inferences justly deducible from them. 
And on the other hand, our leading theorisers in astronomy 
have seldom shown any considerable, or even moderate, 



STAR-CLOUDS AND STAR-MIST. , 339 

powers of observation. It may undoubtedly be ascribed to 
the union of the two qualities in their highest excellence in 
Sir William Herschel, that his labours have influenced in so 
remarkable a manner the progress of modern astronomy, 
and that the praise bestowed upon him of having broken 
down the barriers of the heavens (ccelorum perrnfiit claustra, 
says his epitaph in Upton churchyard) has not been looked 
upon as exaggerated. 

After duly weighing all the circumstances connected 
with the planetary nebulae, this great astronomer at length 
came to the conclusion that they should rather be looked 
upon as presenting the matter from which at some distant 
epoch a single star is to be formed than as consisting of a 
multitude of stars removed to a very enormous distance. 
Strange as the idea may seem at first, it does not seem 
improbable when all the circumstances are considered. If 
we look at our sun and his attendant system of planets, 
and note the brilliancy of his light, the apparent density of 
his mass, and the solidity of the orbs which circle around 
him, it will indeed appear amazing that any could be 
found so bold as to associate that system with a faintly 
luminous globe of gaseous matter such as Herschel sup- 
posed a planetary nebula to be. In every aspect, in bril- 
liancy, in density, in solidity, the contrast seems most 
striking, but when we come to consider the enormous 
volumes of the planetary nebulae, we find that on the lowest 
possible estimate of the density of the gas supposed to form 
them, they must contain an amount of matter fully equal, in 
many instances, to that contained in the whole solar system ; 
and that, therefore, we cannot doubt that from their con- 
densation solid bodies as large as our sun, or larger, might 
very well be constructed. 

A few facts drawn from the calculations of the younger 
Herschel will suffice to show that this conclusion is just. 
There is to be found in the constellation of the Water- 
bearer a planetary nebula with respect to which it has been 
shown that supposing it not farther from us than the nearest 

z 2 



340 MYSTERIES OF TIME AND SPACE. 

fixed star, its globe would fit into the enormous orbit of 
distant Uranus, somewhat as a terrestrial globe within its 
brazen meridian. It follows that this nebula, at the lowest 
computation — for no one doubts that the nebulae are far 
beyond the nearest fixed star — has a volume exceeding the 
sun's nearly seventy thousand millions of times. It would 
follow, of course, that the matter of which it is made must 
be lighter than that which forms the sun, in an exactly 
corresponding proportion, if this nebular globe and our 
sun exactly counterpoised each other. Matter such as this 
would be fifty million-fold lighter than common air ! One 
peculiarity of the planetary nebulae is very striking. It has 
been observed that whereas among the single stars a bluish 
or greenish tint is never noticed, all the coloured stars being 
red, orange, or yellow, several of the planetary nebulae are 
characterised by a very distinctly marked tinge of bluish 
green. We shall presently see the meaning of this pecu- 
liarity. 

For reasons which will appear in the sequel, I mention 
at this stage certain nebulae which astronomers associated 
formerly with the planetary nebulae. These are the singular 
objects called ring-nebulse. In these we see a well-marked 
circular or oval ring of light surrounding a space which is 
either wholly dark or illuminated with a fainter light than 
the enclosing ring. We must also include in the same cate- 
gory the strange object called the dumb-bell nebula, which, as 
seen by Sir William Herschel, presented the figure of an oval 
spot of milky light, enclosing a brighter portion shaped some- 
what like a dumb-bell. 

The last order of nebulae is that from which Sir William 
Herschel derived the strongest arguments in favour of his 
view that a substance of a truly nebulous character, or star- 
mist as some astronomers named the supposed matter, is 
diffused in enormous masses throughout universal space. 
Anyone who looks at the three small stars which form the 
'Sword of Orion' (easily recognised by their position 
beneath the three bright stars of his belt), will notice, if the 



■i 



STAR-CLOUDS AND STAR-MIST. 341 

night be dark and clear, that one of them is involved in a 
faint nebulous light. That light is the famous nebula in 
Orion, which has perhaps attracted more notice among 
observers than any other celestial object, except the sun and 
perhaps the Saturnian ring-system. In a powerful telescope 
this nebula is the strangest and most fantastic object that 
can be conceived. ' Inexplicable to the instructed as to the 
uninstructed eye,' says one who was privileged to examine 
the nebula with the highest telescopic power yet placed at 
man's disposal, ' each can but gaze on those inextricable 
branches and windings, those cloudy masses thinning off 
into the veriest shadows of being, or those other bright but 
comparatively isolated patches lying as it were on the shore 
of absolute blackness ; yes ! forms so mysterious none can 
describe, or while they contemplate them imagine descrip- 
tion to be possible.' In another place, he says, ' How 
gorgeous the brighter parts of the nebula ! How countless 
those streamers branching from it on every side ! How 
strange, especially, that large horn on the north rising in 
relief from the bleak skies like a vast cumulus cloud ! ' 

The Orion nebula may be looked upon as a type of the 
class of objects known as the irregular nebulae. Extending 
over a wide range of sky, over a range which indeed grows 
larger and larger with each increase of telescopic power, 
these objects incalculably exceed all other known objects in 
volume, even if we suppose them to be no farther from us 
than the nearest of the fixed stars. There are not many 
irregular nebulae, yet they probably cover a greater extent 
of the heavens than all the others — six thousand in num- 
ber together. One in the southern heavens, known among 
astronomers as the great nebula in Argo, surpasses the Orion 
nebula in easy visibility. Indeed, from my own observa- 
tions, I can confirm the statement made some ten or 
twelve years ago by Major J. Herschel, in a letter from 
India, that ' the eye catches the nebula as readily as it 
would the Pleiades.' The statement is the more noteworthy 
from the fact that when Sir John Herschel surveyed the 



342 MYSTERIES OF TIME AND SPACE. 

southern heavens the nebula could not be seen with the 
naked eye. It must be mentioned, however, that in that 
day there was a brilliant star shining right through the heart 
of the nebula, which probably dimmed its apparent lustre. 
This star, though still in its old place, is now so much fainter 
as to be barely visible to ordinary eyesight. 

It need hardly be pointed out that the enormous nebulae 
now considered must at once be placed in a class apart from 
others, whatever opinion we might form of their constitu- 
tion. If a star-cluster were to approach so near to us as to 
cover the same extent of sky as the Orion nebula, it would 
blaze with a lustre sufficient to convert night into day. On 
the other hand, if the Orion nebula really consisted of stars, 
placed at so vast a distance as not to be separately visible, 
all the systems of stars we have as yet considered would 
shrink into utter nothingness in comparison with so enor- 
mous a universe. A parallel has sometimes been drawn 
between the great Orion nebula and another known as the 
Andromeda nebula, and also as ' the Transcendently Beau- 
tiful Queen of the Nebulae.' Each is easily visible to the 
naked eye on a clear and moonless night, and each, though 
examined with the largest telescopes ever constructed, re- 
sisted (until quite recent times) all the efforts made by ob- 
servers to resolve them into stars, or at least into discrete 
points of light. But here the resemblance ceases. The 
Andromeda nebula is comparatively regular in figure, is 
very much smaller than the Orion nebula, and there is no 
marked increase in its apparent extent, as larger and larger 
telescopes are directed towards it. We have seen that the 
irregularity of the Orion nebula is its most marked charac- 
teristic, and also that there seems to be no limit to its ap- 
parent dimensions, each increase of light-gathering power 
having brought new branches and outlying streamers into 
view. It was therefore a mistake to suppose that the 
course of discovery respecting the Andromeda nebula could 
in any way illustrate the nature of the Orion nebula. More- 
over, as we shall see, very decisive evidence has been ob- 



STAR-CLOUDS AND STAR-MIST. 343 

tained of a total dissimilarity in the constitution of these 
objects. 

Sir William's Herschel's argument respecting the Orion 
nebula and the other objects of the same class may be 
stated as follows : So long as it is possible to do so, we 
should be led to prefer that interpretation of observed phe-. 
nomena which corresponds most closely with the analogies 
we are familiar with. But in the case of the irregular nebulae 
we must depart from all such analogies. If they are galaxies 
of stars, their dimensions must exceed those of all other star- 
systems taken together ; if their dimensions correspond with 
those of other stellar universes, their constitution must be 
totally different. Having a choice of two views, each of 
which involves a total want of analogy with all that is else- 
where to be observed, that view seems most reasonable 
which does not require us to assign to these objects 
dimensions altogether inconceivable. For although there is 
nothing in the' universe to be paralleled with the irregular 
nebulae, we have instances, even within the range of the 
solar system, of objects (the comets) which are of a totally 
different constitution from the rest of the system they belong 
to. Such, then, the irregular nebulae may be — aggregations 
of some unknown form of matter subserving purposes in the 
economy of nature which are at present altogether unintelli- 
gible to us. They may be constituted of the same material 
as the planetary nebulae, but whereas these, owing to the 
regularity of their figure, may be looked upon as undeveloped 
stars, the wild and fantastic wisps of the irregular nebulae 
afford no intelligible evidence of the processes they are 
undergoing. 

During the first quarter of the present century Herschel's 
views were almost universally accepted among astronomers. 
The skill with which he had prepared the ground for the 
theory, and the address with which he drew illustrations of 
his theory from the number of nebulae which he had ob- 
served, would seem to have forced the new views upon the 
acceptance of his contemporaries. He pointed to nebulae 



344 MYSTERIES OF TIME AND SPACE. 

presenting every variety of structure, and in every stage of 
progress, from the stage of simple globes of star-mist up to 
a state of condensation in which scarcely any nebulosity 
could be perceived. Nay, he went further, and included our 
own sun among the number of condensing systems. Viewed 
from a great distance, he said, there can be little doubt that 
our sun would appear as a star involved in a nebulous light ; 
for the phenomenon called the Zodiacal Light surrounds the 
sun on every side to a distance of seventy or eighty millions 
of miles, and this envelope could present no other than a 
nebulous appearance. 

Time passed, and many theories of Sir William Her- 
schel's, which had seemed bold if not rash at the time of 
their promulgation, had been established on a sure basis by 
the labours of his successors. But his hypothesis of star- 
mist, which had been freely welcomed when it first appeared, 
had not been of the number. c Notwithstanding the inge- 
nuity of the illustrations and incontestable force of reasoning 
by which Herschel sought to establish this bold hypothesis, 
it has not/ said Professor Grant in 1852/ received that con- 
firmation from the labours of subsequent inquirers which 
is so remarkable in the case of many of the speculations of 
that great astronomer. It is now generally admitted that 
the changes which at one time were supposed to be taking 
place in some nebulae were altogether illusive, having been 
suggested partly by erroneous delineation of the objects as 
they actually appeared in the telescope, and partly by the 
different aspects which they assumed when viewed in tele- 
scopes of different degrees of power.' 

It was undoubtedly to the revelations afforded by Lord 
Rosse's great reflector that the disrepute into which Her- 
schel's views had fallen at this time was chiefly due. 

In the first place, the planetary nebulae were found to 
present quite a different aspect in the Parsonstown re- 
flector from that shown by the Herschelian four-feet mirror. 
There is one picture in works on astronomy which attracts 
the attention of the least thoughtful. A strange nebula is 



STAR-CLOUDS AND STAR-MIST. 345 

presented in it, resembling in appearance the face of some 
grotesque animal glaring through a species of halo. This 
object is no other than one of Herschel's planetary nebulae, 
as it appears under the wonderful illuminating power of the 
great reflector. Equally remarkable changes are observed 
in other instances ; and in fact, Lord Rosse stated that ' in 
every instance examined save one the planetary nebulae 
have been found to be nebulae with hollow centres.' By re- 
solving the great ring-nebula- in Lyra into stars — or what 
appear to be such — another blow was dealt to the nebular 
hypothesis. 

But the main contest, as might be expected, was carried 
on around the nebula in Orion. Here the supporters of 
Herschel's views found their chief stronghold. One nebula 
after another had been resolved into stars beneath the pene- 
trating gaze of the great reflector. News came across the 
Atlantic that the magnificent refractor of the Cambridge 
Observatory had, in the clear American skies, reduced 
the great Andromeda nebula into discrete stars. Yet still, 
so long as the Orion nebula remained unresolved, it was im- 
possible to oppugn successfully the views of Sir William 
Herschel. ' With an anxiety natural and profound,' says 
Professor Nichol, ' the scientific world watched the examina- 
tion of Orion with the six-feet mirror, for the result had 
either to confirm Herschel's hypothesis, in so far as human 
insight could confirm it, or unfold among the stellar groups 
a variety of constitution not indicated by those in the neigh- 
bourhood of our galaxy.' 

At length the news came that the nebula had yielded. 
The first account of the resolution of the nebula was con- 
veyed in a letter from Lord Rosse to Professor Nichol. ' I 
think I may safely say,' he wrote, ' that there can be little, 
if any, doubt as to the resolvability of the nebula. Since 
you left us, there was not a single night when, in the absence 
of the moon, the air was fine enough to admit of our using 
more than half the magnifying power the speculum bears. 
Still, we could plainly see that ail about the trapezium is a 



346 MYSTERIES OF TIME AND SPACE. 

mass of stars ; the rest of the nebula also abounding with 
stars, and exhibiting the characteristics of resolvability 
strongly marked.' 

And thus, said astronomers, doubt and speculation 
vanish from this subject for ever. The new fact proves that 
to be real which Herschel deemed incomprehensible. ' And 
now,' — again I quote Professor Nichol, for indeed I am 
here dealing with what others have said, and not with actual 
facts — ' now the astronomer can adduce no justification of 
the assertion that any nebula, however stubborn, ought to 
be interpreted contrary to the analogy of all other known 
objects of its kind.' 

So firmly did the persuasion become fixed in astronomers' 
minds that Herschel had been mistaken, that it is almost 
impossible to take up a book on astronomy written between 
the years 1848 and 1864 without finding attention called to 
the new views which had replaced those he had upheld. 
There were one or two, amongst others the late Admiral 
Smyth, who thought the change of view over-hasty ; but in 
the general rush of opinion against Herschel's theory of star- 
mist, the voices of those who still supported it were lost. 

On a sudden Herschel's opinion was justified. It had 
taken long years of patient observation to overthrow his 
theory, but it was restored to favour by an observation which 
did not occupy five minutes. It is to the spectroscope, the 
most wonderful instrument of modern research, that we owe 
the complete rehabilitation of Herschel's theory of star-mist. 
A very few words will explain the whole matter to readers 
who remember the three fundamental laws of this new mode 
of investigation, viz. : that first, light from a burning solid 
or liquid source gives the rainbow-coloured streak of light 
commonly known as the prismatic spectrum ; secondly, when 
vapours surround such a source of light, the rainbow- 
coloured streak is crossed by dark lines ; and thirdly, when 
the source of light is gas, there is no longer a rainbow- 
coloured streak, but merely a finite number of bright lines. 

Dr. Huggins had been engaged with Professor Miller in 



STAR-CLOUDS AND STAR-MIST. 347 

a careful spectroscopic examination of the brighter fixed 
stars. These physicists had succeeded in establishing a 
series of facts more interesting and surprising, perhaps, than 
anything which had before been revealed to man. 

It occurred to Dr. Huggins that if the new analysis could 
be applied to objects so faint as the nebulae, it could hardly 
fail to afford important revelations respecting their structure. 

The apparatus he made use of was one constructed by 
Mr. Browning, F.R.A.S., the optician, with the express 
object of giving a spectrum of a distinctly marked and 
brilliant appearance. The nebula selected for observation 
was a small planetary one in the Dragon. I quote with a 
few verbal alterations Dr. Huggins's own account of the 
result : ' When I had directed the telescope armed with the 
spectrum apparatus to this nebula,' he wrote, ' I at first sus- 
pected that some derangement of the instrument had taken 
place ; for no spectrum was seen, but only a short line of 
light. I then found that the light of this nebula, unlike any 
other extra-terrestrial light which had yet been subjected by 
me to prismatic analysis, was of definite colours, and there- 
fore could not form a spectrum. A great part of the light is 
monochromatic, and so remains concentrated in a bright 
line occupying a position in the spectrum corresponding to 
its colour. Careful examination showed a narrower and 
much fainter line near the one first discovered. Beyond 
this point, about three times as far from the first line, was a 
third exceedingly faint line.' 

Here, then, at once was the most absolute proof that 
Herschel had been perfectly justified in supposing that the 
planetary nebulae are gaseous. Nothing but a luminous gas 
can give a spectrum of bright lines ; and therefore, so far as 
this question was concerned, no further inquiry was required. 

But before passing on to mention the results obtained 
when other nebulae were examined, we must inquire what is 
the gas of which this nebula is composed ? Had any other 
mode of inquiry but spectroscopic analysis been employed, 
it would have been idle to ask such a question ; but the 



348 MYSTERIES OF TIME AND SPACE. 

wonderful powers of the analysis are equal to answer even such 
a question as this. From the position of one of the bright 
lines, it is inferred that the gas nitrogen is one of the con- 
stituents of the nebula ; another line indicates the existence 
of the gas hydrogen in that far-off system ; the third line 
has not yet been associated with any known terrestrial ele- 
ment, though it is near one belonging to the metal barium 
and still nearer to one belonging to oxygen ; a fourth line 
occasionally seen belongs to hydrogen. 

Dr. Huggins examined a large number of the planetary 
nebulae, obtaining in each case a spectrum which indicates 
gaseity. In some cases only one line could be seen, in 
others two, more commonly three, and in a few instances 
four. When these lines were seen they invariably corre- 
sponded in position with those already described. The 
single line sometimes seen corresponded with the brightest 
line of the three ; and when a second line was visible, this 
also was no new line, but agreed with the second brightest 
line in the three-line spectrum. 

The fourth line was first seen only in the spectrum of a 
very bright small blue planetary nebula, but was later ob- 
served in other cases, and especially in the great Orion nebula. 

So far we see that Sir William Herschel's views have 
been abundantly justified. The planetary nebulae of which 
he said that they are most probably gaseous have been 
shown to be so. The Orion nebula, respecting which he 
expressed a more certain opinion, and on which he founded 
so confidently his much-vexed theory of star-mist, has at 
last supplied confirmatory evidence of Herschel's acumen. 
When Huggins was observing the planetary nebulae, Orion 
was not then visible at night. It was not until several months 
had passed that he was able to apply his spectroscope to 
the analysis of this famous object. We may imagine that 
he came to the examination of the spectrum with some 
anxiety as to the result. * The telescopic observations of 
this nebula/ he says, * seem to show that it is suitable to a 
crucial test of the usually received opinion, that the resolu- 



STAR-CLOUDS AND STAR-MIST. 349 

tion of a nebula into bright stellar points is a certain indica- 
tion that the nebula consists of discrete stars.' A simple 
glance resolved the difficulty. The light from the brightest 
part of the nebula, the very part which under Lord Rosse's 
great reflector blazed with innumerable points of light, gave 
a spectrum identical in all respects with that which Huggins 
had obtained from the planetary nebulae. Thus, what had 
been deemed boldness in Herschel — namely, that he should 
have associated the wildest and most fantastic nebula in 
the heavens with the circular and (in ordinary telescopes) 
almost uniformly luminous planetary nebulae — was unex- 
pectedly confirmed, after the stellar theory of the great 
nebula had been maintained for years, and apparently esta- 
blished by direct observation. 

There, for several years, the matter seemed to rest. In 
1867, it is true, the present Lord Rosse announced that 
parts of the Orion nebula seemed manifestly stellar in con- 
stitution ; but the general result that the great nebulous 
mass is gaseous seemed hardly to be shaken. 

But within the last two years a series of observations 
have been made, marvellous in their nature and promising 
to lead to results throwing entirely new light on the great 
nebula in Orion. A method of research which in former 
times would have seemed utterly impossible — its very con- 
ception so wild and fanciful as to resemble the dream of a 
distempered mind — has been applied, which promises to 
give in the first place far more perfect views of the nebula 
than any yet obtained, and then to give a searching system 
of analysis by which the nebulae may be in the most effective 
manner examined piecemeal, so that varieties in the constitu- 
tion of its various parts may be recognised, if any such exist. 

What would have been thought, even by such men as Sir 
W. Herschel, Laplace, or even Newton, had the idea been 
suggested by which a prepared plate should replace the 
retina of the eye, and instead of the ordinary time for vision, 
an hour or two hours or more should be employed in re- 
ceiving on that artificial retina a picture of a faintly luminous 



35© MYSTERIES OF TIME AND SPACE. 

celestial object ? If the idea could have been for a moment 
regarded as more than a wild and visionary fancy, what 
would even such men have thought if it had been further 
suggested that the artificial eye thus prepared might be 
employed to see rays which the ordinary eye can never see, 
nay, even to determine the nature of the substance from 
which such rays come ? 

Not to deal further with the wonderful nature of the 
photographic method applied to the investigation of the 
Orion nebula — a subject which requires full treatment sepa- 
rately, on account of its own intrinsic importance — I pro- 
ceed to inquire what this method has revealed in regard to 
the question whether the great nebula is or is not a mass of 
self-luminous vapour. 

Dr. Henry Draper, of New York, sent over to England, 
some two years since, enlarged positives from a negative on 
a square inch of glass, in which a wonderful amount of detail 
was shown. In the enlarged positive the exceedingly deli- 
cate details, clearly discernible in the negative, could not be 
perceived, and although, even in these imperfect views the 
features of the nebula could not possibly be mistaken, some 
persons in this country who had tried but failed to accomplish 
what Dr. Draper had achieved, were not above asserting that 
the^markings in the views sent arose simply from imperfec- 
tions in the plate, which chanced to have an arrangement re- 
sembling that of the luminous masses which form the Orion 
nebula. Recently, however, Dr. Draper has taken the only 
worthy revenge he could take for these petty insults ; he 
has obtained still better negatives (for one of them the ex- 
posure lasted no less than two hours and seventeen minutes), 
from which more satisfactory enlargements have been formed 
— and now we have photographs of the nebula which even 
the most envious of unsuccessful rivals have been obliged to 
accept as valid representations — albeit the negatives them- 
selves are far better, nay, are altogether more correct repre- 
sentations of the nebula than the finest drawings yet made 
even with the most powerful telescopes. 



STAR-CLOUDS AND STAR-MIST. 351 

Wonderful, however, though this is, a far more important 
achievement is one which Dr. Draper and Dr. Huggins 
have obtained with equal success — the photographing of the 
spectrum of the great nebula. For thus not only is an ob- 
servation in effect made which can be repeated, as it were, 
by everyone who examines the photographed spectrum, but 
more is thus shown than the eye can see, and moreover 
peculiarities distinguishing one part of the nebula from 
another can be recognised, which, though they might be 
perhaps discernible by the eye, could hardly be regarded as 
demonstrated but for the use of this self-recording method. 

Now here, strange to relate, we get evidence by which 
the conclusion which had been generally accepted is shown 
to be not altogether just. The nebula is seen to be in great 
part gaseous, and where gaseous to shine in the main with 
the tints described above ; but parts of the nebula are not 
gaseous, and those portions which are so are not all con- 
stituted in the same manner. For both Dr. Draper and 
Dr. Huggins hnd portions of the nebula, and especially the 
more condensed parts which lie to the left (in the ordinary 
telescopic, view of the nebula) of that portion which is called 
the Fish's Mouth, to give a continuous spectrum — in other 
words, the same spectrum which we obtain from a star, 1 or 
a star-cluster. This is the spectrum arising from a glowing 
solid or liquid mass, or if from a gaseous body then the 
gaseous body must be in a state of great compression. For 
my own part, indeed, I believe that the rainbow-tinted 
spectrum derived from the Orion nebula by no means indi- 
cates that the source of light is other than gaseous — so that 
we may describe the condensed parts of the nebula in Orion 

1 I say here the same spectrum, though, strictly speaking, the spec- 
trum of a star is not continuous, but consists of a rainbow-tinted streak 
crossed by dark lines due to absorption by the stellar atmosphere. But 
with the open slit used by Drs. Draper and Huggins to obtain the pho- 
tograph of the spectrum of the Orion nebula, such dark absorption 
bands could not possibly be seen. They are not seen, in fact, in the 
spectra of star-clusters. 



352 MYSTERIES OF TIME AND SPACE. 

as shining with lustre indicating stellar or sun-like constitu- 
tion. We may then find here the explanation of the bright 
points of light seen by Rosse in the brighter parts of the 
Orion nebula — in these parts the gaseous matter would seem 
to have already aggregated into stars or suns. 

But the stars thus forming must be immersed in the 
glowing gas forming the general substance of the nebula. 
We might infer this independently of observations tending 
actually to prove it. For it would be absurd to suppose 
that the nebula is a flat surface, constituted in one way in 
one part and otherwise in another. We must always re- 
member this in observing objects like comets, nebulae, star- 
clusters, and so forth. We have a view of them from one 
side. Viewed from any other direction they would appear 
differently, no doubt ; but they would still present the same 
general characteristics. We often find them spoken of as 
if they were regarded as flat objects which we see from in 
front, or square to their surface, so that if viewed edgewise 
they would appear as mere lines. But, when we find parts 
of the nebula in which stars seem to be congregated, with 
nebulous matter above, below, and on either side in the 
field of view, we must remember that nebulous matter lies 
also in all probability (certainly, one might fairly say) 
between us and the stellar aggregation, as well as on the 
farther side. 

As evidence of this, I may note that when in Dr. 
Draper's photographic experiments the slit of the spectro- 
scope was placed across the trapezium, one of the hydrogen 
lines shown in the spectroscope was of the same length as 
the slit, a duplication of effect being noticed where it inter- 
sected the continuous spectrum of the trapezium stars. On 
this Dr. Draper remarks that, ' if this effect is not due to 
flickering motion in the atmosphere ' (an explanation which 
may be at once dismissed, as the effect would certainly 
have been recognisable in that case all along the bright 
lines), * it would indicate that hydrogen gas was present 
even between the eye and the trapezium,' 



STAR-CLOUDS AND STAR-MIST. 353 

Varieties of gaseous constitution are also indicated in 
different parts of the nebula. ' In the case of two other 
faint lines in this vicinity/ says Dr. Draper, speaking of the 
brighter part of the nebula, ' I think the lines are not of 
the length of the slit, one being quite short and the other 
discontinuous. If this observation,' he proceeds, ' should 
be confirmed by future photographs of greater strength, 
it might point to a non-homogeneous constitution of the 
nebula, though differences of intrinsic brightness would 
require to be eliminated.' 

Again, variety is indicated (a different kind of variety) 
■by the differences between the photographs obtained by Dr. 
Draper and Dr. Huggins. Speaking of a bright line, not 
visible in the ordinary spectrum (belonging in fact to the 
ultra-violet portion), which is shown conspicuously in Dr. 
Huggins's photographs, Dr. Draper remarks that he has not 
found that line, though his photographs show other lines 
which Dr. Huggins does not appear to have photographed. 
Dr. Huggins says, indeed, of other lines which he expected 
to find, ' if they exist in the spectrum of the nebula, they 
must be relatively very feeble : I suspect, indeed, some very 
faint lines in this part of the spectrum ' (between the most 
refrangible of the lines visible to the eye and the line just 
referred to), ' and possibly beyond the more conspicuous 
photographic line. I hope by longer exposures and with 
more sensitive plates to obtain information on this and 
other points,' Dr. Draper considers, and it seems to me 
justly, that the difference between the results obtained by 
himself and Dr. Huggins may be due to the fact that the 
slit had been placed on different regions of the nebula, 
though part of the difference may have resulted from Dr. 
Huggins's employment of a reflector and a prism of Iceland 
spar, whereas Dr. Draper used a reflector and a prism of 
flint glass. 

It is manifest that we have in the application of photo- 
graphy to the gaseous nebulae a new and potent means of 
research. A remark applied by Dr. Huggins to the use of 

A A 



354 MYSTERIES OF TIME AND SPACE. 

photography in obtaining star-spectra, may probably be 
applied with even more force here. ' We shall, perhaps, 
underrate/ he says, ' the importance of a knowledge of the 
ultra violet ' part of spectra, ' if we regard these photographs 
as simply adding so much in length to the visible spectrum ; 
for there are reasons why a knowledge of this part of the 
spectrum may be of exceptional value to us.' If, as is pro- 
bable, the luminosity of the gaseous portion of the Orion 
nebula is accompanied by but a relatively small proportion 
of heat, then the rays from the violet and ultra-violet part 
of the spectrum are likely to give us much more complete 
information respecting the constitution of these nebulous 
masses than can be derived from the visible part of the 
spectrum. Still, it is well to remember that should the 
ultra-red part of the spectra of nebulae be able to convey 
information supplementing or adding new force to informa- 
tion otherwise derived, we may entertain good hopes that 
that part also of the spectrum may be effectively studied. 
The researches and discoveries of Captain Abney, though 
they have not yet shown how the ultra-red rays may be 
used as effectively (for rapidity of action, &c.) as the ultra- 
violet, have at least shown that they can be used ; and we 
may well believe that the methods which, even now, when 
this department of the photographic art is in its infancy, 
have enabled Captain Abney to photograph a kettle of 
boiling water in the dark by means of its invisible heat 
radiations, may hereafter be developed to give photographs 
of the ultra-red spectra of stars, and to show lines (if such 
exist) in the ultra-red part of the spectra of the gaseous 
nebulae. When we remember, however, that there are no 
visible lines in the yellow, orange, or red, we may reason- 
ably doubt whether any exist in the ultra-red. 

Certainly Dr. Huggins's remark seems thoroughly jus- 
tified by what has been already done in this direction. ' It 
is not, perhaps, too much to hope,' he says, ' that the further 
knowledge of the spectrum of the nebulas afforded us by 
photography, may lead, by the help of terrestrial experi- 



STAR-CLOUDS AND STAR-MIST. 355 

ments, to more definite information as to the state of things 
existing in those bodies.' 

The inquiry may prove to be far more wide-reaching 
than those might imagine who view it merely as relating to 
the question how the gaseous or partly gaseous nebulae 
are constituted. It may well chance, as long since sug- 
gested by Prof. Clark of Cincinnati, and as more cautiously 
hinted by Dr. Huggins, that in the varieties of constitution 
observed in the irregular nebulae and the evidence such 
varieties afford of progressive change, we may find not 
merely direct evidence of the development of suns and 
sun-systems from great masses of nebulous matter (the 
luminous star-mist of the Herschelian theory), but even 
what would be a far more important and impressive result 
— actual evidence of the development of the so-called 
elements from substances really elementary, or, at any rate, 
one stage nearer the elementary condition than are our 
hydrogen, nitrogen, oxygen, carbon, and so forth. The 
peculiarity of the spectral indications of the presence of 
nitrogen and hydrogen in the nebulae is, that only one line 
of nitrogen and two or three lines of hydrogen are dis- 
cernible, instead of the complete spectrum of either element 
as seen under any known conditions, seems suggestive of 
what may be called a more elemental condition of hydrogen 
and nitrogen. If it should prove hereafter that in different 
parts of the great nebula of Orion (or the inquiry may be 
more successfully pursued with the great nebula in Argo), 
hydrogen and nitrogen can be traced in gradually varying 
forms, up to those giving the complete spectrum at some 
known pressure and temperature, we should not only be 
able to form some conception of the actual condition of 
various portions of the great nebulous mass, not only be 
able to recognise something as to the progressive aggregation 
of different portions of the mass into suns and sun-systems, 
but we should learn something as to the conditions under 
which hydrogen and nitrogen, and perhaps other so-called 
elements, are formed. Nor can I doubt that, should this 



356 MYSTERIES OF TIME AND SPACE. 

prove to be the case, the inquiry would presently be found 
associating itself with those other inquiries by which Ruther- 
furd, Secchi, Draper, Huggins, and others have been led to 
recognise the existence of progressive stages in the develop- 
ment of suns themselves. If, for instance, the stars can be 
arranged in a series whose first term is represented by the 
blue-white stars, of which Sirius and Vega are the type ; 
the second by yellowish stars, of which Capella and our 
own sun are typical ; the third by orange stars, like 
Arcturus ; and the fourth by red stars ; and if of these the 
bluish-white are the youngest (in development, of course ; 
we speak not of absolute age), and the others more and more 
advanced, we may well believe that the careful study of 
objects like the Orion nebula, with the new means now 
available, may bring us to the knowledge of yet earlier terms 
in the series, indicating the various steps by which gaseous 
matter aggregates into embryonic suns, these into bantling 
orbs, which develope (as time-periods, measureless by man, 
pass onwards), into suns like Sirius and Vega in growth, and 
thence to the condition which our own sun has attained. 

In conclusion, I would note how abundantly the diverse 
views, about which I spoke at starting, and the inquiries to 
which those rival views led, have justified Herbert Spencer's 
teaching that no answer was ever yet given by science which 
did not lead to new and closer questioning. On one side 
and on the other the controversy swayed as fresh evidence 
on either side was obtained ; but science was not content 
at any stage of the inquiry to 'rest and be thankful.' Now, 
when so much new knowledge has been obtained, and 
when so many doubtful points have been disposed of, 
we are further than ever from actually understanding the 
mystery of the great gaseous nebulas. But if we find more 
than ever about which we are in doubt, we see more than 
ever how much fresh knowledge may be hoped for as we 
push forward new inquiries. 



357 



HERBERT SPENCER'S PHILOSOPHY. 



There are many who find a difficulty in understanding how 
the philosophy of Mr. Herbert Spencer is related to the 
Darwinian theories of biological evolution. Many, indeed, 
seem to find difficulty in recognising at all the nature of the 
teachings of Mr. Spencer, and especially in determining the 
position which they hold in modern thought. Some appear 
to imagine that his views are entirely sociological, others 
suppose that they involve simply an extension of the Dar- 
winian doctrine to the universe at large, while yet others (as 
I have repeatedly noticed in converse with those whom I 
have met during my lecturing tours in this country, America, 
and Australasia) appear to regard Mr. Spencer as chief 
among the opponents of religion. 

It should hardly be necessary to say that all these views 
are erroneous \ yet knowing as I do, how few there are who 
have formed any just conception of Mr. Spencer's philosophy, 
especially in this country (for he is much better under- 
stood and appreciated on the other side of the Atlantic), I 
have seen, somewhat gladly, that certain unfair treatment 
in ' Reminiscences chiefly of Oriel College,' by the Rev. 
Thomas Mozley, has led to the publication by Mr. Spencer 
of a succinct statement of the cardinal principles involved 
in the successive works which Mr. Spencer has published, 
The statement is a mere summary, technically, and in some 
places, rather obscurely worded ; but it is of great value, as 
showing not only what Mr. Spencer has actually taught, but 
what it has been his special purpose to teach. I propose 



358 MYSTERIES OF TIME AND SPACE. 

now to translate the successive items of this statement into 
more familiar language (in each case giving Mr. Spencer's 
actual words in the first instance). As, however, the signifi- 
cance of a statement of this kind must always in part depend 
on the circumstances which elicited it, I deem it well briefly 
to sketch the matter at issue between Mr. Spencer and the 
Rev. Mr. Mozley. I do this the more willingly, that, as 
the former remarks, ' serious injustice is apt to be done by 
the publication of reminiscences which concern others than 
the writer of them ; and widely diffused as is Mr. Mozley's 
interesting work, his statement will be read and accepted by 
thousands who will never see "Mr. Spencer's Rectification." ' 
It appears to me that good service will be done to the cause 
of justice by helping to spread this rectification as widely as 
possible. 

The passage which has called forth Mr. Spencer's recti- 
fication runs as follows : ' I have indulged,' says Mr. Mozley, 
'from my boyhood in a Darwinian dream of moral philo- 
sophy, derived in the first instance from one of my early 
instructors. This was Mr. George Spencer, (honorary) sec- 
retary of the Derby Philosophical Association, founded by 
Dr. Darwin, 1 and father of Mr. Herbert Spencer. My dream 
had a certain family resemblance to the " system of philo- 
sophy " bearing that writer's name. There was an important 
and saving difference between the two systems, between that 
which never saw the light, and perished before it was born, 
without even coming to wither like grass on the housetops, 
and that other imposing system which occupies several yards 
of shelf in most public libraries. The latter makes the world 
of life, as we see and take part in it, the present outcome of 
-a continual outcoming from atoms, lichens, and vegetables, 
bound by the necessities of existence to mutual relations, 

1 It was 'more than a dozen years,' Mr. Spencer remarks, 'after 
Dr. Darwin's death in 1802, when my father became honorary 
secretary. I believe my father (who was twelve years old when Dr. 
Darwin died) never saw him, and so far as I know, knew nothing of 
his ideas.' 



HERBERT SPENCER'S PHILOSOPHY. 359 

up to or down to brutes, savages, ladies, and gentlemen, 
inheriting various opinions, maxims, and superstitions. The 
brother and elder philosophy, for such it was, that is mine, 
saved itself from birth by its palpable inconsistency, for it 
retained a Divine original, and some other incongruous 
elements. In particular, instead of rating the patriarchal 
stage hardly above the brute, it assigned to that state of 
society a heavenly source, and described it as rather a 
model for English country gentlemen, that is, upon the 
whole, and with certain reservations.' 

It will be tolerably obvious that in this passage there is 
something more than Mr. Spencer — proceeding in his calm 
way by inquiring rather what others found in it than what 
he found himself— notes as its purport. It leaves the im- 
pression, he says, that the doctrines set forth in the system 
of Synthetic Philosophy, as well as those which Mr. Mozley 
entertained in his early days, were in some way derived from 
the elder Spencer. True, but it leaves also the impression 
that although the ' brother and elder philosophy had been 
thus derived/ it owed to Mr. Mozley whatever development 
it received ; he speaks of it plainly as the ' philosophy that 
is mine.' It conveys very clearly (and also very cleverly) 
the idea that in Mr. Mozley's opinion the elder philosophy 
•was altogether the nobler and better of the twain, however 
obvious it may be to sounder judgments that that opinion is 
altogether erroneous. And then, by saying that even this 
elder and better philosophy was so palpably unsound that 
its failure before birth saved it from its due fate, it leaves 
us clearly to understand what a great misfortune, in Mr. 
Mozley's eyes, has been the birth, growth, and development 
of the younger and inferior brother. That these palpable 
sneers (not to say these gross insults) escaped an attention 
so keen as Mr. Spencer's I do not suppose. It is evident, 
however, that he very justly regarded them as unworthy of 
notice — they are, in fact, of the class of innuendoes which 
may properly be described as womanish (observe, I do not 
say womanlike). Mr. Spencer directs his whole attention 



360 MYSTERIES OF TIME AND SPACE. 

to meet Mr. Mozley's implication that during the last five- 
and- twenty years he has been allowing himself to be credited, 
with ideas which are not his own. ' Since this is entirely 
untrue/ he says, ' I cannot be expected to let it pass un- 
noticed ; if I do I tacitly countenance an error, and tacitly 
admit an act by no means creditable to me.' 

He then tells us, in admirably selected terms, just how 
far he believes himself to be indebted to his father. His 
indebtedness was general, he says, not special — and in- 
debtedness for habits of thought encouraged rather than 
for ideas communicated. c I distinctly trace to him an in- 
grained tendency to inquire for causes — causes, I mean, of 
the physical class/ And here let me note in passing, is the 
great lesson which modern science is ever inculcating. It 
is here that science influences mental and moral culture 
most palpably. There is no more valuable safeguard against 
superstitions of all orders, from those which affect the whole 
conduct of life, the whole character, down to the paltry 
superstitions which relate to such matters as helping to salt, 
walking under ladders, and so forth, than the inquiry always 
for causes. Breaking a mirror means seven years of sorrow, 
says the ignorant believer in foolish fancies of the sort : c In 
what way ? through what relation of cause and effect ? ' comes 
the question of common sense, and the notion is at once 
seen to be an absurdity. If I commit such and such offences, 
says the believer in a higher form of superstition, I shall be- 
punished ; science asks how and why, and in the answer 
finds the real reason for the moral law. Science finds that 
offences against right and justice bring always their punish- 
ment with them, and shows cause why ; establishing thus a 
sounder and nobler morality than any founded on the merely 
superstitious fear that some unexplained punishment will falL 
on us for wrong-doing. 

The elder Spencer, says his son, was ' far from having- 
himself abandoned supernaturalism, yet the bias towards, 
naturalism was strong in him, and was, I doubt not, com- 
municated (though rather by example than by precept) to* 



HERBERT SPENCER'S PHILOSOPHY. 361 

others he taught, as it was to me. But while admitting, and 
indeed asserting, that the tendency towards naturalistic in- 
terpretation of things was fostered in me by him, as probably 
also in Mr. Mozley, yet I am not aware that any of those 
results of naturalistic interpretation distinctive of my works 
are traceable to him.' 

What is specially noticeable about this part of Mr. 
Spencer's communication to the Athenceinn, as bearing on 
the nature of the Spencerian philosophy is, that he shows 
Mr. Mozley's statement to be necessarily erroneous. The 
cardinal ideas discussed throughout the series of volumes 
published by Mr. Spencer were necessarily of much later 
origin than the period to which Mr. Mozley's account refers. 
The great generalisation respecting the correlation and 
equivalence of the physical forces, had not as yet been even 
thought of. This doctrine and others which are absolutely 
indispensable elements of the general theory of evolution, 
were not heard of for years after the time mentioned by 
Mr. Mozley, so that, unless his statement be regarded as 
simply untrue, it is manifest that he has (even now, when 
Mr. Spencer's views have long been before the public) no ade- 
quate conception even of the general doctrines of evolution 
as formulated in our time, far less of those special doctrines 
with which Mr. Herbert Spencer's name is identified. 

Nay, it will be manifest to all who have carefully studied 
Mr. Spencer's own writings, that he himself had not adopted 
the views now forming the Spencerian philosophy until long 
after his father's death. ' In the earliest of them,' he says, 
1 " Letters on the Proper Sphere of Government," published 
in 1842, and republished as a pamphlet in 1844, the only 
point of community in the general doctrine of evolution is 
a belief in the modifiability of human nature through adap- 
tation to conditions (which I held as a corollary from the 
theory of Lamarck), and a consequent belief in human pro- 
gression. In the second and more important one, " Social 
Statics," published in 1850, the same general ideas are to 
be seen, worked out more elaborately in their literal and 



362 MYSTERIES OF TIME AND SPACE. 

political consequences. Only in an essay published in 
1852 would the inquirer note for the first time a passing 
reference to the increase of heterogeneity as a trait of de- 
velopment, and a first recognition of this trait as seen in 
other orders of phenomena than those displayed by indi- 
vidual organisms. Onwards through essays published in 
several following years, he would observe further extensions 
in the alleged range of this law, until, in 1855, in the " Prin- 
ciples of Psychology," it begins to take an important position, 
joined with the additional law of integration, afterwards to 
be similarly extended. Not until 1857, in two essays then 
published, would he find a statement relatively crude in 
form, of the law of evolution, set forth as holding through- 
out all orders of phenomena, and joined with it the state- 
ment of certain universal physical principles which necessitate 
its universality, And only in 1861 would he come to an 
expression of the law approximating in definiteness to that 
final one reached in 1867.' This, of course, conclusively 
disposes of the implication in Mr. Mozley's statement ; for, 
were this true, the earlier writings would have contained 
traces of the doctrines set forth in the later ones. 

So much premised, we are prepared to examine Mr. 
Spencer's summary of the cardinal principles developed in 
his successive works. So far as the refuting of Mr. Mozley's 
unfair implication is concerned, nothing more could be 
needed ; but Mr. Spencer thought it might be well to 
enable the reverend gentleman, who had somewhat rashly 
-attacked him, to compare the propositions of * the younger 
philosophy ; with that which, because — by his own account 
— it was unborn at an earlier date, Mr. Mozley has called 
*the elder.' The summary was written out some twelve 
years ago, for an American friend. Let us take the propo- 
sitions seriatim. 

1. Throughout the universe in general and in detail, there 
is an unceasing redisti'ibution of matter and motion. 

This statement scarcely needs explanation. It means 
simply that the arrangement of matter in space, whether we 



HERBERT SPENCER'S PHILOSOPHY. 363 

consider large masses or small, is ever varying, and that the 
movements among the different portions of matter, large as 
well as great, are constantly changing in rate and in direction. 
In the way of illustration we have a wide field for selection. 
Amid stellar space stars or suns are rushing hither and 
thither with motions varying according to the attractions 
at work in various parts of each star's course. We see 
that in vaporous matter, or widely distributed cosmical dust, 
similar motions are continually taking place. The expan- 
sive tendencies of gaseous matter are found to be due to the 
rush of minute molecules in all directions. So that, in the 
universe of matter as revealed to the astronomer, the same 
general laws affect the suns which people space, the largest 
discrete masses we know of, and the molecules which form 
the intimate substance of gaseous matter, bodies so minute 
as to lie hopelessly beyond the range of any conceivable 
increase of microscopic power. In organic matter the same 
law holds. All the physical processes which are most ob- 
vious and familiar, all those which form the subject of the 
most recondite scientific research, are in reality illustrations 
of the constant redistribution of matter and motion. 

2. This redistribution constitutes evolution where there is 
a predominant integration of matter and dissipation of motion, 
and constitutes dissolution where there is a predominant ab- 
sorption of motion and disintegration of matter. 

Unfortunately the words in this statement are not alto- 
gether well chosen or used in their strictly correct sense — 
evolution is set against dissolution as if the two were con- 
trasted processes, whereas dissolution is a form of evolution. 
Moreover, the word ' integration ' is not commonly under- 
stood, whether in its technical sense (which is almost purely 
mathematical) or in ordinary language, in the sense in which 
it is here used. It is rather understood ordinarily to mean 
the restoration of that which had been made imperfect, 
than as the converse of disintegration. 1 'Absorption' also 

1 This also is the sense in which the Romans used the word inte- 
gratio. Thus the familar saying of Terence, Amantium im amoris 



364 MYSTERIES OF TIME AND SPACE. 

is here used by Mr. Spencer to mean what might rather 
be described as * assumption.' The statement may be thus 
translated into ordinary language : — 

This constant change in the distribution of matter and 
motion, results in some cases in the aggregation into one 
whole, of portions of matter which before had been apart 
from each other, the motions of these several portions inter 
se coming to an end, or greatly diminishing, as they thus 
gather into a single mass. In such cases we have the 
formation of new masses. In other cases portions of matter 
which had been aggregated into a single mass are separated 
from each other, and begin to move freely inter se. In such 
cases we have the dissolution of the masses thus separated 
into their component parts. 

We may select our first illustrations of these converse 
processes from the celestial spaces, though so far as is yet 
known these tell us of few cases of dissolution or dissipation,, 
nearly all the processes actually observable being instances 
of aggregation or of the formation of new masses. We know 
that in the solar system there are multitudinous systems of 
meteoric bodies — and we know further that our own earth 
gathers in many millions of these bodies in each year. The 
same is doubtless true of the moon, Venus., Mars, and other 
planets. It must be true in yet greater degree of the sun. 
Every fall of a meteor is a process of aggregation. When a 
meteor falls on the earth a new mass is formed out of the 
two — the earth and the meteor — which had before existed 
apart. On the other hand, it has been thought possible, 
some think it has been proved, that meteors were originally 
projected from the sun or planets — if so, every such case- 
was an illustration of the process of segregation ; masses 
which had before formed a single mass, and had shared the 
same motions in reference to other masses, being thrown 
apart to move thenceforth for a longer or a shorter period 

integratio est, is properly translated, ' The quarrels of lovers are the- 
renewing of love.' 



HERBERT SPENCER'S PHILOSOPHY. 365 

of time (in some cases for ever) independently of each 
other. If Mr. George Darwin's views are correct, and our 
Ttioon was at any time separated either in a single mass (or, 
.as I think more probable, a ring of small masses) from the 
earth, that was a process of dissolution, or, rather, segre- 
gation. So also with the other satellite systems, and with 
the rings of Saturn, if they were really formed in the way 
-suggested. In the latter case these rings illustrate at the 
same time both processes ; for while they were on this 
hypothesis expelled from Saturn, and so illustrate segrega- 
tion, their component small masses are gathering together 
to form hereafter a single moon, or it may be two or three 
moons, and-thus illustrate aggregation. 

But to show the generality which characterises Mr. 
Spencer's theories we may here take an illustration of a 
quite different character. In the growth of a nation out 
of tribal races which had been pursuing each a separate 
existence, nomadically traversing a continent before they 
settled down to occupy a country, we have a process of 
aggregation ; in the growth of colonies formed out of groups 
of persons who have left their country at different times and 
with diverse aims and purposes, we have also a process of 
.aggregation in one aspect, for these colonies are formed by 
the aggregation of groups before moving apart ; but in a 
wider aspect we have segregation, each colony being sepa- 
rated by the removal of parts from what had before been one 
whole nation. Where a colony is formed of groups from 
several nations, there is, in reference to the colony, a pre- 
dominant aggregation of matter and dissipation of motion ; 
where one nation forms several colonies there is, in re- 
ference to the nation, a predominant segregation of matter, 
and assumption of motion. 

But, in fact, as these processes take place throughout the 
universe in general and in detail, we might select our illus- 
trations from a thousand different sources ; we might view 
matters on a very large scale or on a very small scale, or on 
a scale having any position between these extremes. 



366 MYSTERIES OF TIME AND SPACE. 

The third and fourth statements of Mr. Spencer's phi- 
losophy run thus : — 

3. Evolution is simple when the process of integration, or 
the formation of a coherent aggregate, proceeds uncomplicated 
by other processes. 

4. Evolution is compound when, along with this primary 
change from an incoherent to a coherent state there go 011 
secondary changes due to differences in the circu7?istances of the 
different parts of the aggregate. 

The explanation of 2 leaves little to be explained here. 
The reader must note carefully, however, that the word 
evolution here has that limited and incorrect sense in which 
it has already been used in statement 2. It means here the 
process of formation of a coherent aggregate out of matter 
which had before been scattered more or less sparsely. 
Probably no illustration can be given from nature of an 
absolutely simple process of aggregation any more than 
illustrations can be given from nature of perfectly straight 
lines, circles, ellipses, and so forth. The philosopher may 
speak of simple evolution and define it, just as the Newtonian 
may speak of motion in a circle around a central attracting 
mass, but as a matter of fact no such evolution can take place. 
In every instance of change from an incoherent to a co- 
herent state, that is, from the condition of matter more or 
less scattered to matter forming a single aggregate, changes 
arising from the different circumstances of the different 
portions of aggregating matter must in every case occur. 
From the nebulous mass aggregating into a system of suns, 
down to the aggregation of the minutest drop or vesicle of 
fluid from vapour, or even in all probability down to the 
formation of molecules out of atoms (only no one knows 
what atoms or molecules in reality are), there must be 
varieties of condition in the forming mass causing differences 
of constitution in the mass formed ; at any rate I find 
myself unable, after long study, to think of any case 
in which an absolutely uniform process of aggregation 
takes place, or can ever be conceived to take place. Of 



HERBERT SPENCER'S PHILOSOPHY. 367 

compound processes, of course the illustrations are endless. 
Those we have already considered will serve as well as 
any others. Every process of formation, or of what Mr. 
Spencer here understands by the word evolution, must of 
necessity be varied in different parts of the forming aggrega- 
tion by varieties in the conditions under which the formative 
process is- applied to different parts of the aggregating 
material. Whether it be a stellar system forming out of 
star mist, or a solar system out of meteoric and cometic 
matter, or a single sun, or other celestial body out of 
multitudinous bodies before discrete — or, again, whether 
the aggregation is utterly unlike any studied by the 
astronomer, as the aggregation of a nation out of many 
races, or the formation of a society of any sort from 
scattered and before unassociated individuals, or the 
growth of an animal by the slow building together 
of material particles drawn from many sources, or the 
development of a race of animals by any process of 
evolution: — in every case, the various parts of the aggregate 
must differ inter se, because of the varying conditions 
under which they are severally formed. 

5. These secondary changes constitute a transformation of 
the homogeneous into the heterogeneous — a transformation 
which, like the firsts is exhibited in the universe as a whole and 
in all {or nearly all) its details ; in the aggregate of stars and 
nebulce, in the planetary system ; in the earth as an inorganic 
mass ; in each organism, vegetal or animal ( Von Baer's law 
otherwise expressed); in the aggregate of organisms through- 
out geologic time ; in the mind; in society ; in all products of 
social activity. 

This amounts to the statement that processes of aggre- 
gation, affected as they are by varieties of condition, result 
in variety of structure. I venture to object to the statement 
that the homogeneous is transformed into the heterogeneous, 
because it implies that homogeneity can for a time (however 
brief) exist. This is contrary to experience. The process 
which actually takes place is a transformation from the less 



368 MYSTERIES OE TIME AND SPACE. 

heterogeneous to the more heterogeneous, the strengthening 
or stronger marking of varieties of structure which began 
with the very beginning of the forming aggregation. Abso- 
lute uniformity never has existed or can exist in any part of 
the universe, large or great, any more than any absolute 
physical entity can exist which answers to the geometrical 
definition of a straight line. But the importance of the 
proposition before us is not modified by this necessary 
change. It remains true, that in the processes by which 
coherent aggregates of any sort whatever, and on whatever 
scale, are formed out of materials before separate and dis- 
crete, varieties which began at the very outset of the process 
become more and more marked as the process continues. 

As an illustration, consider the formation of the solar 
system. We need not trouble ourselves to decide between 
the various theories which have been formed as to the actual 
way in which the solar system came into existence. Suffice 
it that whether the system was formed by the contraction ol 
a mighty mass of nebulous matter, or by the aggregation of 
meteoric matter, the circumstances under which the plane- 
tary scheme came into existence were originally such that the 
growth of some — as of Jupiter and Saturn — was encouraged, 
while others, like Mars, Mercury, and the asteroids, acquired 
bulk and substance with difficulty. From the beginning 
the former planets were doubtless the larger ; and as time 
went on, their absolute, and probably their relative, superi- 
ority increased, until at length they came to occupy their 
present position, that is, to use the technical language which 
seems unfortunately coming into vogue in this subject, they 
became differentiated from the minor planets by their 
marked superiority in bulk and mass. 

The law thus recognised prevails everywhere throughout 
the universe, in general as in detail. It is the physical form 
of the law, ' To him that hath shall be given, and from him 
that hath not shall be taken, even that which he seemeth to 
have.' Thus, in the case just considered, to the planets 
which had large bulk and mass, greater bulk and mass were 



HERBERT SPENCER'S PHILOSOPHY. 369 

given, while those which were smaller, lost, through the 
relative slowness of their growth, even that position which 
originally no doubt they seemed to have as members of 
a family of bodies not altogether unlike, and fell into a 
different and (so far as age is concerned) an inferior class. 
It is the same in all cases of systematic evolution. We 
are thus led to the law indicated in Mr. Spencer's next 
statement, the wording of which, like that of some which 
follows, is not altogether calculated to invite the attention of 
the unscientific reader, though the subject matters of Mr. 
Spencer's philosophy are such that all should understand its 
general purport. 

6. The process of integration, acting locally as well as 
generally, combines with the process of differentiation, to 
render the change described in 5 not simply one from homo- 
geneity to heterogeneity, but from an indefinite homogeneity, to 
a definite heterogeneity ; and this trait of increasing definite- 
ness, which accompanies the trait of increasing heterogeneity, is, 
like it, exhibited in the totality of things, and in all its 
divisions and subdivisions, down to the minutest. 

In plain English this might be expressed somewhat as 
follows : — 

When a whole is forming, the various parts not only 
differ because they are formed under different conditions, 
but because the formative process itself (acting on the 
various parts as well as on the forming mass or system as a 
whole) tends to produce different results in different parts. 
Thus not only is the product varied in character in its 
different parts, but the differences are definite. There is 
not an indefinite gradation from one form to another, but 
distinct steps of gradation so to speak, — and this is recog- 
nised throughout the universe, regarded as a whole, and in 
all its parts, and in all the parts of these parts, down to the 
minutest subdivisions. 

For instance, in the solar system, the formation of a 
single system out of indefinitely distributed and moving 
matter, led to the formation of a central mass, and of bodies 

B B 



370 MYSTERIES OF TIME AND SPACE. 

travelling around that mass, distinctly unlike it in character. 
Among the bodies thus travelling the formative process, 
acting diversely in different parts, and having varied quan- 
tities of matter and rates of motion to deal with, formed 
diverse families of bodies, the great planets in one group, 
the minor planets in another, the asteroids in a third—all 
definitely distinguished from each other. In biological 
evolution definite variations taking their origin at first in 
minute differences of condition, surroundings, and so forth, 
separate the various races, animal and vegetable : in each 
race the various genera become similarly distinct, while in- 
dividual members of the same genus are also distinguished 
one from the other by peculiarities arising from the different 
constitutions under which each is formed. The same again 
is seen in the formation of distinct nations from among the 
various tribes and races of men, the characteristics which dis- 
tinguish nation from nation becoming more and more marked 
as the nations gradually gather coherence and what may 
be called national individuality. Within one and the same 
nation class distinctions arise and become more and more 
marked with the progress of time. Within each class minor 
distinctions come into existence, and separate sub-class 
from sub-class more and more definitely. Still smaller sub- 
divisions are formed, which in turn become more and more 
characteristically distinguished one from the other, till we 
reach the family, and finally the individual members of the 
family— the limits of subdivision in this direction. And so 
in every possible case, under all conceivable conditions, on 
the large scale as on the small, the law holds — aggregation 
is inevitably accompanied by the appearance of varieties of 
condition, quality, &c, in the parts of the aggregating mass, 
and, as aggregation proceeds, these varieties become more 
and more marked. 

7. Along with this redistribution of the matter compos- 
ing any evolving aggregate there goes on a redistribution of the 
retained motion of its components in relation to one a?iother ; 
this also becomes, step by step, more definitely heterogeneous. 



HERBERT SPENCER'S PHILOSOPHY. 371 

As the aggregate is gradually formed, the motion of the 
matter forming the aggregate is gradually dissipated, as 
when masses which had been travelling freely around the 
sun or a planet are one by one brought to rest on the 
surface of the body whose mass they thus help to build up. 
But the motion is not altogether lost. It may, as in the 
case just considered, result in motion affecting the formed 
mass as a whole — the rotation in this case of a sun or a 
planet. It may result in systematic movement within the 
scheme or system thus formed, as in the movements of 
the planets within the solar system, or of satellites within a 
system circulating round a planet. But in the Spencerian 
philosophy motion is used in a wider sense. Thus the 
formation or evolution of a race of animals involves a dissi- 
pation of motion — the tendency to irregular changes being 
resolved into systematic variation — freedom to vary in 
any direction, merging gradually into the tendency to 
change only in specific directions and according to uniform 
law. So with other cases, even less like mere physical pro- 
cesses of aggregation, as we see in national, municipal, and 
social groupings. The law for each aggregate becomes 
more and more definite for each as time passes, precisely as 
the aggregates themselves become so. 

We now come to statements belonging to the a priori 
aspect of the subject. It is evident that the only way in 
which we can conceive an utter absence of all tendency to 
the redistribution of matter and motion is by conceiving 
perfect uniformity throughout the entire universe : then and 
then only would all matter be related to all other matter in 
a manner absolutely indifferent, so that there would be no 
tendency either to aggregation or to any change in motions 
already existing, or to the state of absolute rest (if such were 
the condition of the primary and absolutely uniform uni- 
verse thus conceived). Thus we have, next, the statements 
that, 

8. In the absence of a homogeneity that is infinite and 
absolute, the redistribution, of which evolution [formation of 



372 MYSTERIES OF TIME AND SPACE. 

aggregates] is one phase, is inevitable. The causes which 
necessitate it are, — 

9. The instability of the homogeneous which is consequent 
upon the different exposures of the different parts of any 
limited aggregate to incident forces. The transformations 
hence resulting are complicated by, — 

10. The multiplication of effects. Every mass and part of 
a mass on which a force falls, subdivides and differentiates 
thai force, which thereupon proceeds to work a variety of 
changes ; and each of these becomes the parent of similarly 
multiplying changes, the multiplication of them becoming greater 
in proportion as the aggregate beco7?ies more heterogeneous. 

It will be observed that Mr. Spencer, in statement 8, 
recognises that only infinite and absolute uniformity could 
produce absolute stability. It appears to me that the same 
condition is required in order that evolution should be 
simple. For wherever, in the neighbourhood of any forming 
mass, or at whatever distance from it, there is a want of 
uniformity, the circumstances are not such that simple 
evolution can result ; and we can imagine no circumstances 
in which, however remote might be the region where abso- 
lute uniformity ceased, the effect of such homogeneity 
would not in the long run be felt, resulting in what Mr. 
Spencer has described as compound evolution. 

In statement 9 Mr. Spencer points out that uniformity 
in itself is essentially unstable. That which is uniform in 
structure inevitably tends to become diverse in structure 
when it is exposed to diverse conditions. The slightest 
breath of air will ripple the surface of level water, while 
powerless to affect the onward course of a wave. It is so 
throughout nature. Opposing forces may result in a con- 
dition of stable equilibrium ; absolutely uniform conditions 
are, of their very nature, unstable. 

In like manner, statement 10 needs little explanation 
and no proof. It is evident that in every process of evo- 
lution the various forces which produce various effects must 
be infinitely varied in their operation, according to the con- 



HERBERT SPENCER'S PHILOSOPHY. 373 

dition of the various parts of the aggregating whole. With 
every variation of their effects, the condition of that aggre- 
gating whole varies further. The variations thus arising 
may be cumulative in some parts or self-correcting in others, 
whence come into existence regions of greater variety and 
regions tending to such uniformity as results from counter- 
poised variations. Thus the aggregate becomes more and 
more varied in detail as well as in general — these sub- 
regions, so to speak, of uniformity dividing off regions of 
diversity. We may again use as an illustration the effect 
of winds upon the sea. The surface which had been uniform 
becomes uneven under the diverse action of the wind on 
various parts. Afterwards the wind, as it falls on the waves 
which traverse the water's surface, is modified in direction 
by their resistance and, being deflected in various ways, 
tails yet more diversely than before on the different parts of 
the water-surface : hence arises another kind of diversity, a 
minor order of varieties, which varieties in turn produce 
other and yet smaller forms of variety — the number of 
changes thus resulting being continually greater and greater 
as the surface becomes more and more disturbed. 

We might find illustrations of this law in the star-depths, 
in the formation of a planetary system, in the shaping of 
such a world as our own earth. But we have illustrations 
more immediately interesting in relation to the general 
doctrine of evolution. What Darwin defines as the com- 
plex relations of animals and plants to each other in the 
struggle for existence affords an admirable illustration of the 
diversity of effects resulting from the inter-relation of varied 
action and varied condition in that which is acted upon. 
Consider, for instance, the following passage in the ' Origin 
of Species ' : — ' In several parts of the world insects deter- 
mine the existence of cattle ; perhaps Paraguay offers the 
most curious instance of this; for here neither cattle nor 
horses nor dogs have ever run wild, though they swarm 
northward and southward in a feral state ; and Azara and 
JReugger have shown that this is caused by the greater 



374 MYSTERIES OF TIME AND SPACE. 

number in Paraguay of a certain fly, which lays its eggs in 
the navels of these animals when first born. The increase 
of these flies, numerous as they are, must be habitually 
checked by some means, probably by other parasite insects. 
Hence, if certain insectivorous birds do decrease in Paraguay,, 
the parasite insects would probably increase, and this would 
lessen the number of the navel-frequenting flies — then the 
cattle and horses would become feral, and this would cer- 
tainly greatly alter (as, indeed, I have observed in parts of 
South America) the vegetation ; this again would largely 
affect the insects ; and this, the insectivorous birds, and so 
onwards in ever-increasing circles of complexity. Not that 
under nature the relations will ever be as simple as this. 
Battle within battle must be continually recurring with 
varying success ; and yet, in the long-run, the forces are 
so nicely balanced that the face of nature remains for long 
periods of time uniform ' [that is, with such uniformity as 
results from omnipresent variety] though assuredly ' the 
merest trifle would give the victory to one organic being 
over another.' 

We are led directly to recognise among the causes of 
increasing variety : — 

1 1. Segregation, which is a process tending ever to separate 
unlike units, and to bring together like units — so serving con- 
tinually to sharpen, or make definite, differentiation otherwise 
caused. 

As the result of these processes, a balancing of the forces 
at work arises from the matter worked on assuming those 
conditions which best favour their existence. The waves 
on our illustrative sea came to have just dimensions and 
just periods of oscillation on the greater scale, and they in 
turn are traversed by minor waves, and these by wavelets, 
and these in turn by ripples, harmonising with the winds 
and the variations of the winds which originally produced 
them. Thus, passing from the illustration to the processes 
of evolution illustrated. 

1 2. Equilibration is the final result of these tra7isforma- 



HERBERT SPENCER'S PHILOSOPHY. 375 

tions which an evolving aggregate undergoes. The changes go 
on until there is reached an equilibrium between the forces 
which all parts of the aggregate are exposed to and the forces 
these parts oppose to them. Equilibration may pass through a 
transition stage of balanced motions (as in a planetary system), 
or of balanced /mictions (as in a living body), on the way to 
ultimate equilibrium ; but the state of rest in inorganic bodies, 
or death in organic bodies, is the necessary limit of the changes 
constituting evolution. 

This tendency to uniformity, really arising as a result of 
the constant subdivision and multiplication of diversities is 
seen on the largest scale (known to us) in the generally 
recognised tendency of this universe of ours to that con- 
dition of uniform temperature which would constitute its 
death, and on the smallest scale, in the natural death of 
animals and plants or of parts of these. Our sun is alive so 
long as, being of a higher temperature, he communicates 
heat to what lies around him ; the stellar system is alive so 
long as some of its constituent parts are at higher levels of 
energy than the rest — just as a sea is active when the various 
parts of its surface are at different levels. The animal body 
is alive so long as the diverse energies of its various parts 
result in the processes of circulation and respiration. With 
uniformity resulting from the subdivision and distribution of 
energy comes death. But after death come processes akin 
to renewed life ; though no longer the same life. 

13. Dissolution is the counter-change which sooner or later 
every evolved aggregate undergoes. Reinaining exposed to sur- 
rounding forces that are unequilibrated, each aggregate is ever 
liable to be dissipated by the increase, gradual or sudden, of its 
contained motion; and its dissipation, quickly undergone by 
bodies lately animate, and slowly undergone by inanimate 
masses, remains to be undergone at an indefinitely remote 
period by each planetary and stellar mass, which since an 
indefinitely distant period in the past has been slowly cooling ; 
the cycle of its transfortnation being complete. 

We can of course only infer from analogy that the 



376 MYSTERIES OF TIME AND SPACE. 

heavenly bodies and systems of bodies after the equilibra- 
tion of their energies with their surroundings — after, for 
instance, each sun has exhausted its superior heat, and 
therefore no longer ceases to part with heat to surrounding 
matter — will undergo a process of dissolution, thus com- 
pleting the cycle of its transformations. It is so, we see, on 
the minor scale in every case with which we can deal. It 
is so with the individual members of animal and vegetable 
races, with families of animals and vegetables, with groups 
of these families, with nations, with social organisations. In 
every case we see how the life of each aggregate is limited 
in time, and tends to death, but how, also, after death the 
parts of the aggregate are dissolved, and become ready to 
take part in the formation of other aggregates. Hence — 

14. This rhythm of evolution and dissolution, completing 
itself during short periods in small aggregates, and in the vast 
aggregates distributed through space completing itself in periods 
which are immeasurable by huma7i thought, is — so far as we 
can see, universal and eternal — each alternating phase of the 
process predominating now in this regio7i of space and now in 
that, as local conditions determine. 

From the cases considered in the last paragraph, we can 
proceed with a certain degree of confidence to cases more 
extended, until we recognise in the solar system (for in- 
stance) the evidence of youth, and life, and old age, as 
stages of evolution, — though our processes and the range of 
our observation are too limited to enable us to judge (other- 
wise than from analogy) that after old age and death in these 
vast physical aggregations there comes a stage of dissolution 
completing the cycle of transformations. Ail we can say on 
that point is that, as in every case we can deal with to the 
end, we have found dissolution following the state of equi- 
librium which we call death in the case of the individual 
and compare to death in other cases, so also it is with those 
cases which (because of the limited range of our vision) we 
can only deal with in minute parts. We judge then that 
planets after this stage of death have a stage also of disso- 



HERBERT SPENCER'S PHILOSOPHY. S77 

lution, though no physical experience enables us to say 
what that stage is like. We pass also to higher orders of 
being. We see suns of various ages throughout stellar 
space, and learn to recognise in their case also progression 
and evolution, up to and beyond the fulness and prime of 
stellar life. In their case also must come death, with equi- 
libration between their energies and the receptive capacities 
of matter around them ; and after this physical death must 
come, though in ways we cannot perceive, a process of dis- 
solution, completing the cycle of transformations. So also 
with higher orders, with systems of suns, with systems of 
such suns, and so on, absolutely without end. 

We come then to the final statements respecting the 
operations of nature and their significance. After what has 
been already explained, these need no words of mine to 
make them clearer. Nor could this paper be better closed 
than in the very words of this great teacher of our age : — 

15. All these phenomena, from their great features even to 
their minutest details, are necessary results of the persistence of 
force, tender its forms of matter and motion. Given these as 
distributed through space, and their quantities being unchange- 
able either by increase or decrease, there inevitably result the 
continuous redistributions distinguishable as evolution and 
dissolution, as well as all those special traits above enumerated. 

16. That which persists, unchanging in quantity, but ever 
changing i?i form under these sensible appearances which the 
universe presents to us, transcends human knowledge and con- 
eeption, is an unknown and unknowable power, which 

WE ARE OBLIGED TO RECOGNISE AS WITHOUT LIMIT IN SPACE, 
AND WITHOUT BEGINNING OR END IN TIME. 



378 MYSTERIES OF TIME AND SPACE. 



A SURVEY OF THE NORTHERN 
HE A VENS. 

The ideas entertained by the non-scientific public respect- 
ing the extent and nature of the researches made by- 
astronomers into the constitution of the heavens, are, for 
the most part, singularly inaccurate. Many suppose, for 
instance, that the astronomer knows the distances of at 
least those stars which are visible to the naked eye, and 
they hear with surprise that there are not six stars of the 
millions which astronomy really has to deal with whose dis- 
tances can be regarded as even approximately determined. 
It is commonly supposed, again, that the whole surface of 
the celestial sphere has been so surveyed with powerful 
telescopes that astronomers know according to what laws 
the stars are distributed over the heavens. Few, indeed, 
even among those who may be called students of astronomy 
are aware how very far this idea is from the truth. We hear so 
often of the star-gaugings of the Herschels, and the results 
inferred from those star-gaugings are so confidently insisted 
upon in our text-books of astronomy, that when the actual 
extent of the Herschelian gauges is mentioned, the student 
is apt, in his sense of surprise and disappointment, to un- 
dervalue the labours of the Herschels as unduly as he 
had before exaggerated their extent and importance. Yet 
another mistake is commonly made. The range of the 
telescopes employed by astronomers is compared with the 
range of unaided vision, and men are apt to suppose that 



A SURVEY OF THE NORTHERN HEAVENS. 379 

the astronomer obtains the same insight into the consti- 
tution of the heavens as though his powers of vision were 
correspondingly increased. The fact is forgotten that the 
telescope can only show the astronomer a minute portion of 
the heavens at a single view ; that the information it is 
capable of supplying is in a sense piecemeal ; and that the 
real lessons taught by the telescope, so far as the distri- 
bution of the heavenly bodies is concerned, can only be 
learned by combining together a number of large-scale 
views of separate portions of the heavens, into a single com- 
paratively small-scale picture. It is not known that our 
materials for this work are for the most part incomplete, 
and that in those few instances where we have complete 
materials little has been done to utilise them. 

I propose in this paper to consider, briefly, the extent of 
the researches hitherto made into the subject of the consti- 
tution of the star-depths ; and then to describe some of the 
conclusions which seem deducible from an inquiry I have 
recently instituted, with the object of presenting in a single 
picture the results of one of the noblest series of labours yet 
undertaken by astronomers : Argelander's complete survey 
of the northern heavens with a telescope showing stars 
down to the tenth magnitude. Setting aside surveys limited 
to small regions of the heavens, it may be said that the only 
observational labours yet directed to the solution of the 
noblest physical problem man can study, are the star- 
gaugings of the Herschels. These star-gaugings constitute 
in reality but a minute proportion of the work achieved 
by these great astronomers. Yet, we may justly say, with 
Struve, that not one of the feats undertaken by the Herschels 
surpassed in daring that of attempting to gauge the star- 
depths with a telescope a foot and a half in aperture. If 
an astronomer devoted all his observing hours to such 
work, he would need, on a moderate computation, full 
eighty years to complete the survey of the heavens. 1 It is, 

1 ' Supposons que Ton puisse faire, pendant ioo nuits de l'annee, 



380 MYSTERIES OF TIME AND SPACE. 

therefore, with no thought of undervaluing the credit due to 
the Herschels for their star-gaugings, but solely because it 
is important that true ideas should be held on the subject, 
that I now refer to the relatively small extent of the heavens 
actually included in their survey. 

Sir William Herschel published, in 1785, the results of 
no less than 3,400 star-gauges. Each star-gauge gave the 
number of stars visible in the field of view of Herschel' s 
telescope, — this field being equal in extent to almost exactly 
one-fourth of the apparent dimensions of the moon's disc ; 
and in the course of these surveys — setting aside rough 
estimates on which Herschel himself laid no stress,— about 
90,000 stars were counted. 

At first sight we seem to have here a widely extended 
survey. The portion of the heavens actually gauged was 
equal to an extent which it would require 800 moons to 
cover ; and thirty times as many stars as can be seen with 
the naked eye in the darkest and clearest night were actually 
counted. But when we leave the region of ordinary notions 
— whether as respects the moon's apparent size or the 
multitude of the stars — and enter into the real particulars, 
the startling nature of the disproportion between the extent 
of the heavens and the portion surveyed by Herschel is at 
once recognised. It is calculable that each field of view 
surveyed by Sir William Herschel amounted to but the 
£3 2, 9 7 9th part of the celestial sphere. So that, in fact, he 
had gauged but about the 250th part of the area of the 
heavens. To obtain a clear idea of the minuteness of this 
proportion, suppose for a moment that the whole surface of 
the heavens is represented by an ordinary chess-board \ then 
the combined extent of all the gauge-fields of Sir William 
Herschel would amount to barely the fourth part of one of 
the black or white squares of such a board. 

•chaque nuit 100 jauges, et il ne faudra pas moins de 83 ans pour 
le jaugeage du ciel entier. En effet, les 3,400 jauges de Herschel 
forment un de ses travaux les plus hardis. ' Struve's Etudes d'Astronomie 
stellaire, note 74. 



A SURVEY OF THE NORTHERN HEAVENS. 38] 



But even this illustration affords but an imperfect idea 
of the actual and admitted incompleteness of Herschel's 
table of star-gauges. One must turn to the table itself, to 
recognise at once the limited area of the field Herschel 
surveyed, and the difficult conditions under which such 
surveys are conducted by the astronomer. The great object 
Herschel had in view was, the determination of the re- 
lative number of stars visible in different directions ; and it 
was obviously essential to his purpose that the conditions 
under which the gauges were made should be as nearly 
constant as possible. Yet we find that he notes opposite 
hundreds of his gauge-fields, that the observation was 
marred, either by haze, or moonlight, or twilight, or even by 
strong daylight, or by the light of the Aurora Borealis, or, 
lastly, by the low position of the gauged region. 

As regards the number of stars counted by Sir William 
Herschel, we may recall to mind that, according to his com- 
putation, the Milky Way probably contains 18,000,000 stars 
visible in the 18-inch telescope he employed ; while the in- 
defatigable Struve considers that probably there are more 
than 20,000,000 such stars in the whole heavens. Thus we 
see that the 90,000 stars actually counted are relatively few, 
however large the number may seem by comparison with 
the number of stars visible to the unaided eye. 

As respects Sir John Herschel's labours in the same 
field, it is only necessary to note that they were less ex- 
tensive than his father's ; the number of gauges amounting 
to 2,300, and the number of stars actually counted amount- 
ing to about 70,000. 

In all, the star-gauges of the Herschels extended over 
the 150th part of the heavens, and included 160,000 stars 
actually counted. Let me repeat that I am far from desiring 
to undervalue these labours. The very fact that, among all 
the hundreds of astronomers who have studied the heavens 
with powerful telescopes, not one has yet rivalled either of 
the Herschels in this work of star-gauging, shows how 
highly the labours of those great astronomers should be 



382 MYSTERIES OF TIME AND SPACE. 

esteemed. But I hold it to be essential that just ideas 
should be entertained on subjects of this sort. It is as un- 
wise as it is wrong in principle to describe in misleading 
terms the labours of our great men, or to forget — as if due 
honour to their memory required such forgetfulness — to 
institute a searching comparison between the work they 
effected and what yet remains to be achieved. It is for- 
tunate that men like the Herschels have ever been the last 
to encourage exaggerated estimates of their labours — inso- 
much that we find Sir William Herschel speaking of his 
star-gauging as intended only ' as an example to show the 
spirit of the method ' — while Sir John Herschel again 
and again insisted on the necessity of fresh and more 
extended surveys than he and his father had been able to 
undertake. 

The researches of the Herschels being the only ob- 
servational labours directed by astronomers to the actual 
survey of the heavens with reference to the laws of stellar 
distribution, we have only to consider the inquiries directed 
to the analysis of those observations, in order to ascertain 
the sum and substance of the information actually ob- 
tained respecting the constitution or architecture of the 
sidereal system. When we inquire who are the astronomers 
who have attempted the task of educing from the labours 
of the Herschels their true value, we find that we have only 
to add the name of William Struve to the names of the 
Herschels themselves, in order to complete the list of such 
inquirers. Of course I am well aware that many others 
have discussed the works of the Herschels, and yet more 
have described the results to which the work has been 
held to point. But besides the elder Struve and the Her- 
schels themselves, not one astronomer has subjected the 
observations of the latter to thorough investigation and 
analysis. 

Now, there is one peculiarity in the several inquiries of 
the elder and younger Herschel, which deserves very careful 
consideration. Sir John Herschel unquestionably supposed, 



A SURVEY OF THE NORTHERN HEAVENS. 383 

when discussing his own observations, that they confirmed 
the views of the elder Herschel — these views being those 
enunciated by Sir William Herschel in the famous paper of 
1785, in which the star-gauges above referred to were first 
fully discussed. And, certainly, the statistical evidence ob- 
tained by Sir John Herschel corresponded very closely with 
that obtained by his father. But the remarkable point is, 
that Sir William Herschel had virtually abandoned the views 
of 1785, and that Sir John Herschel seems either not to 
have known, or to have forgotten, the circumstance ! This 
must appear incredible, I am aware, to most of my 
readers, if not to all ; more especially as all the text-books 
of astronomy agree in describing the views of Herschel 
in 1785 as those which he maintained to the close of his 
career ; or rather, these books fail to mention that Sir 
William Herschel had in any respect modified his views. 
I feel that it will be a somewhat difficult task, under the 
circumstances, to establish the fact that the theory of T785 
was actually abandoned by the elder Herschel ; but yet 
the evidence on the point appears to me so convincing, 
when rightly apprehended, that I shall at least claim the 
careful attention of the reader while I enunciate its leading 
features. 

In the first place, in order to remove what must appear 
as a strong antecedent improbability in my view of the 
matter, let me remind the reader of the length of time which 
had elapsed between the death of the elder Herschel and 
the epoch when the younger Herschel undertook the supple- 
mentary series of star-gaugings in the southern heavens. 
The last of Sir William Herschel's papers on the general 
subject of the construction of the heavens appeared in the 
Philosophical Transactions for 1818, thirty -tJu-ee years, let 
me note in passing, after the enunciation of the cloven disc 
theory of the sidereal system. Six years later, or in 1824, 
the elder Herschel died. Now the star-gaugings of the 
younger Herschel were not commenced until the year 
1834, ten years after Sir William's death ; nor have we any 



384 MYSTERIES OF TIME AND SPACE. 

evidence that the younger Herschel's attention had been 
directed in a special manner to the subject of the star- 
gauges, either during the life of Sir William Herschel or 
afterwards during the progress of Sir John Herschel's ob- 
servations in England. The eight years devoted by the 
younger Herschel to preparation for his observations at the 
Cape, were employed in the study of double stars, in the 
examination of nebulae, and in other work of like nature, 
not in the comparatively rough work of star-gauging. At 
the Cape, even, star-gauging held a quite subordinate posi- 
tion in Herschel's labours. Here are the words in which he 
refers to the subject, in the noble work wherein he gives an 
account of his observations. After mentioning that his 
father's labours in 1785 had led to the theory that the 
sidereal system forms a stratum ' bifurcated or spread out 
into two sheets,' he proceeds : — ' It may easily be supposed 
that the opportunity of carrying out this great induction by 
observations made with the same telescope, similarly used 
in that part of the heavens inaccessible to its author, was 
not neglected. So soon as a knowledge of the regions where 
nebulse might more especially be expected had been ac- 
quired, so as not to hazard too much by continually inter- 
rupting the sweep for the purpose of gauging, a system of 
star-gauges was set on foot, at first somewhat irregularly, 
but after a very few sweeps, more systematically, so as to 
dot over the heavens, as it were, with a regular tesseration of 
gauged or counted fields, disposed at definite and equal 
intervals.' Then presently he mentions that ' many gauge 
points would needs escape observation, in consequence of 
the interference of the more important regular business of 
the sweep.' We find also that blanks were left where the 
heavens ' had been sufficiently swept over before the gauges 
commenced, or where, for other reasons, their registry was 
interrupted.' 

So far as can be judged, then, the attention of Sir John 
Herschel was first specially directed to the subject of star- 
gauging at this epoch of 1834, ten years after his father's 



A SURVEY OF THE NORTHERN HEAVENS. 385 

death, and half a century after the enunciation of the cloven 
disc theory of the sidereal system. 1 

For the sake of brevity, I will select the two most marked 
features of the theory of 1785 ; and I shall be able, I think, 
to show that Sir William Herschel adopted later altogether 
different opinions on both points. 

It will not be questioned by those who are acquainted 
with the theory of 1785, or with what may be called the 
text-book theory of the sidereal system, that according to 
this theory the Milky Way is a congeries of stars, resembling 
our sun and the stars visible to the naked eye, and dis- 
tributed in a similar manner throughout that portion of space 
which is occupied by our stellar system. Now, seventeen 
years later Herschel wrote as follows : — 'Though our sun 
and all the stars we see may truly be said to be in the plane 
of the Milky Way, yet I am now convinced, by a long in- 
spection and continued examination of it, that the Milky 
Way itself consists of stars very differently scattered from 
those which are immediately about us.' 

Again, it will not be questioned that Sir William Herschel, 
* when dealing with the star gauges of 1785, held richness of 
condensation to imply an enormous extension of the star- 
system in the direction where such richness was recognised. 

1 If it should be objected, notwithstanding, that Sir John Herschel 
could hardly have forgotten any essential circumstances connected 
with a theory which his own father had enunciated, I would answer 
that, admitting those circumstances to have been fully known to him 
during his father's lifetime — which is not proved (and is even improb- 
able in my judgment) — convincing evidence may yet be cited to show 
that Sir John Herschel might have forgotten them ten years after his 
father's death. For, in the Preface to the fifth edition of his Outlines 
of Astronojuy, Sir John Herschel comments on certain researches of 
M. Reynaud as novelties, though, as he noted afterwards, he had 
himself formerly reasoned out the same subject to precisely similar 
conclusions — a fact which ' had completely escaped his recollection,' 
he says, ' when perusing the work of M. Reynaud.' Every student of 
science must, we conceive, have met with instances in his own ex- 
perience rendering forgetfulness of this sort far from surprising. 

c c 



386 MYSTERIES OF TIME AND SPACE. 

This was, indeed, the essential principle of the method of star- 
gauging, the fundamental law on which that system was based. 
Yet, in 1802, Sir William Herschel thus absolutely negatives 
this rule of interpretation. After citing, as a convenient illus- 
tration of his new views, the two great clustering aggregations 
of the Milky Way between the stars Beta and Gamma, of the 
constellation Cygnus, he proceeds : ' These milky appear- 
ances deserve the name of clustering collections, as they are 
certainly brighter about the middle and fainter near their 
undefined borders. For in my sweeps of the heavens it has 
been fully ascertained that the brightness of the Milky Way 
arises only from stars ; and that their compression increases 
in proportion to the brightness of the Milky Way. We may, 
indeed, partly ascribe the increase both of brightness and of 
apparent compression to a greater depth of the space which 
contains these stars ; but this will equally tend to show their 
clustering condition ; for, since the increase of brightness is 
gradual, the space containing the clustering stars must tend 
to a spherical form if the gradual brightness of the stars is 
to be explained by the situation of the stars.' This passage, 
written as it is in Herschel's somewhat condensed style, ' 
may require paraphrasing ; but there can be no real question 
as to its meaning. What Herschel says amounts to this : 
the rich region with its bright centre and undefined borders 
is a clustering collection ; it may be that the richness of the 
central portion is due altogether to real richness of the cluster 
there ; but even admitting that the extra richness at the 
centre is partly due to greater range of the space containing 
the stars, yet still these stars must be held to form a cluster 
of a globular form ; for even if the gradual increase of bright- 
ness is to be thus explained by the situation of the stars, yet 
the way in which the increase takes place serves to prove 
that the space containing the clustering stars must tend to a 
spherical form. This is the obvious meaning of the passage ; 
and this interpretation of the appearances in Cygnus and 
elsewhere is wholly incompatible with the theory of 1785. 
According to this theory, the rich regions of the Milky Way 



A SURVEY OF THE NORTHERN HEAVENS. 387 

corresponded to places where the stellar system extended to 
a vast distance, the richness in the telescopic field being due 
to the fact that the range of the instrument passed along an 
enormously extended array of stars reaching from the sun's 
neighbourhood to the extreme limits of the Milky Way. 
According to the views of 1802, the rich regions of the 
Milky Way correspond to regions within the star-system 
where real clusters exist — somewhat spherical in figure, 
perhaps, so that the line of sight through the middle of a 
cluster traverses a longer range than lines of sight towards 
the edge — but certainly true clusters, rich aggregations 
separated from surrounding space by a comparatively void 
region all around them. According to one view, the Milky 
Way might be compared to a fog surrounding the observer, the 
directions where the fog seems densest being those in which 
its extension from the observer is really greatest ; according 
to the other view, the Milky Way is to be compared to a 
bank of more or less closely set clouds, some of which — 
corresponding to the clustering aggregations in the Milky 
Way — can be distinctly recognised apart from the others. 
According to one theory, there is a continuous star-connec- 
tion, so to speak, between the sun and the Milky Way — rich 
regions and poor alike ; according to the other, there is no 
such connection ; but the line of sight directed to the rich 
clustering aggregations of the Milky Way traverses, before 
reaching them, a relatively barren tract. 

I have here taken the ''two most salient features of the 
theory of 1785, and have shown that these features are not 
only wanting in the views of 1802, but are replaced by fea- 
tures of a quite different character. I will now show that 
an astronomer who undoubtedly gave to the works of the 
elder Herschel a closer scrutiny than any other (not except- 
ing Sir John Herschel) has cared to give, arrived at the 
conviction, not only that the theory of 1785 had been en- 
tirely abandoned by Sir William Herschel, but that the two 
main principles on which that theory had been based were 
given up by Herschel. 

c c z 



388 MYSTERIES OF TIME AND SPACE. 

The elder Struve, in his admirable work, entitled Etudes 
<F Astronomic stellairc, gives an excellent digest of the papers 
of Sir William Herschel. It is well to note the circumstances 
under which Struve studied Herschel's papers, for otherwise 
the full importance of his conclusions will not be recognised. 
He had already given the papers one reading, and had failed 
to notice how the views of the elder Herschel changed 
between 1785 and 1802. 1 

This is not surprising, when we remember that Herschel's 
papers are written in a somewhat terse style, and that when 
exhibiting new views he does not point out where and how 
these differ from opinions he may have before expressed. 
When we consider that the papers written by Herschel range 
over nearly half a century, it will be understood that they do 
not form a perfectly congruous series ; and at the same time 
we can understand that the progression and change of ideas 
would not, at a first reading, be rightly apprehended. I 
certainly found this to be the case in my own first study of 
Herschel's papers ; insomuch that I have written many 
passages in books and essays as though the views I advo- 
cated were opposed to his, when in reality they were accord- 
ant with his later opinions. And I believe that no one, 
unadvised of Herschel's change of view, and studying his 
papers from beginning to end with an ordinary degree of 
attention, would arrive at any other conclusion than that to 
which Struve came in the first instance, and that which I 
conceive Sir John Herschel entertained to the end of his 
life. But in 1830 (let the date be noted in connection with 

1 This is to be inferred from the following passage. After speaking 
of his own earlier researches, he notes that the facts he is now about 
to present must be regarded as ' une discussion ulterieure a laquelle je 
me suis engage, surtout par une nouvelle etude des memoires de Sir 
W. Herschel, faite depuis, et dont les resultats ont ete exposes dans la 
partie historique de ce rapport. En effet, je me suis convaincu que 
mes idees actuelles sur la voie lactee ne sont en opposition qu'avec le 
systeme de Herschel de 1785, mais extremement conformes aux vues 
posterieures de ce grand astronome.' 



A SURVEY OF THE NORTHERN HEAVENS. 389 

what has been remarked respecting the younger Herschel's 
star-gauging) Struve received from Sir John Herschel a 
valuable present — a complete copy of the elder Herschel's 
papers, the author's original manuscript enriched by his 
corrections and by several manuscript notes. It was this 
noble gift that led Struve to the thorough investigation (a une 
etude assidue) of the works of the elder Herschel. 

Struve shows clearly that the theory of 1785 is based on 
two hypotheses, the failure of either of which (by Herschel's 
own admission) would render the theory inadmissible. The 
first is the hypothesis of a generally uniform distribution 
of stars throughout the sidereal system ; the second is the 
hypothesis that the gauging telescope was capable of pene- 
trating to the limits of the sidereal system in all directions. 
Then Struve quotes passage after passage belonging to the 
papers of 1802 and later years, in which these hypotheses 
are definitively abandoned by Sir William Herschel; and 
he arrives at this noteworthy conclusion, that the theory of 
1785, respecting the constitution of the Milky Way, is com- 
pletely negatived by the later researches of its author, and 
that Herschel himself had completely abandoned it. l 

Yet Struve very properly combined the observations of 
1785 with his own researches. Clearly recognising the 
limited range of those observations and the necessity for 
further survey of the heavens, he yet took advantage of the 
numerical statistics they afforded to supplement the relations 
he was the first to discuss ; those, namely, which are to be 
recognised in the statistical distribution of stars of the higher 
or brighter orders of magnitude. 2 

1 Struve's words are as follows : ' Nous parvenons done au resultat 
— peut-etre inattendu mais incontestable — que le systeme de Herschel, 
enonce en 1785, sur 1' arrangement de la voie lactee, s'ecroule de toutes 
parts, d'apres les recherches ulterieures de Pauteur ; et que Herschel 
iui-meme Fa entierement abandonne. ' 

2 There is some confusion of expression as to the different orders 
of star magnitude. Some astronomers speak of the fainter stars as 
belonging to the higher orders, because larger numbers are employed 



390 MYSTERIES OF TIME AND SPACE. 

Struve's researches were carried out with the express 
object of determining whether the arrangement of stars of 
the brighter orders of magnitude affords any evidence re- 
specting the constitution of the sidereal system. Piazzi had 
noticed that even among the stars visible to the naked eye 
there is a tendency to aggregation towards the Milky Way : 
a phenomena clearly irreconcileable with the theory of 1785. 
For, according to this theory, the stars are spread with a 
certain general uniformity throughout the stellar system, 
and the richness of the Milky Way is due to the greater 
extension of the system in the direction of a certain stratum. 
Within this imagined stratum, a cross -section of which 
illustrates Herschel's paper, a sphere enclosing all the stars 
visible to the unaided eye would be completely enclosed, or 
rather, immersed. Herschel tells us, indeed, the proportion 
which he conceived to exist between the dimensions of his 
section and the extent of his sphere of naked-eye stars, or 
lucid stars, as they are called for distinction. The section 
is 10 j inches long and 2\ inches wide, and he remarks that 
this section ' is drawn upon a scale where the distance of 
Sirius is no more than the 80th part of an inch ; so that 
probably all the stars, which in the finest nights we are able 
to distinguish with the naked eye, may be comprehended 
within a sphere drawn round the large star near the middle, 
representing our situation in the nebulae of less than half a 
quarter of an inch in radius ; ' that is a sphere separated by 
a distance of more than nine times its own radius from the 
flattened sides of the imagined stratum, and by more than 

to indicate those orders. Others— and, on the whole, their plan seems 
preferable— speak of the fainter stars as belonging to the lower orders 
of magnitude. So far as analogous numerical instances are in question, 
both methods can be justified ; for the mathematician speaks of equa- 
tions of high order when the number expressing the order is large ; 
and, on the other hand, we commonly speak of persons or objects as 
low down in a list when the number expressing their place is large. 
Since the fainter stars are inferior in the primary quality of a star— that 
is, lustre — it would seem that those orders which are expressed by the 
larger number, being thus inferior, should be called the lower orders. 



A SURVEY OF THE NORTHERN HE A PENS. 391 

forty times that distance from the rim of the stratum. It is 
quite obvious that the construction of this small sphere could 
be in no way associated with the construction of the stratum, 
if the theory were just, any more than the disposition of the 
water vesicles within a space a foot in diameter in the heart 
of a cloud could be associated with the figure of the cloud. 

Struve's first inquiries were applied to stars down to the 
seventh magnitude inclusive. He showed that these stars 
are not spread in a uniform manner over the heavens, but are 
more densely aggregated upon and near the zone of the 
Milky Way. The method of research he employed was 
perhaps not altogether satisfactory, since he merely counted 
the number of stars included severally, according to his 
catalogue, in the twenty-four equal spaces into which the 
hour-meridians l divide a series of zones parallel to the celes- 
tial equator. The spaces were too large to afford more 
than a rough estimate of the laws of distribution. But still 
there was so obvious an increase of richness when the Milky 
Way was approached, so marked a poverty near the poles of 
the galaxy, that Struve was able to announce with confidence 
that the theory of 1785 was unsound. 

When a careful re- examination of Sir W. Herschel's 
papers had shown him that Herschel had himself abandoned 
the theory of 1785, Struve undertook a new series of re- 
searches . He now employed a much larger number of stars 
than he had before dealt with. M. Weisse had reduced and 
arranged the stars observed by Lalande, Bessel, and Arge- 
lander, so far as a zone of the heavens, thirty degrees wide, 
was concerned. This zone, ranging from 15 degrees north 
to 15 degrees south of the equator, contained no less than 
31,085 stars, from the first to the ninth magnitude, inclusive; 
of these 1664 belonged to the first six orders of magnitude, 
2,500 were of the seventh magnitude, 8,183 °f tne eighth, 
and 19,738 of the ninth. But all the stars of these orders, 
and belonging to the zone, were not included in Weisse's 

1 Circles through the poles of the heavens, dividing the celestial 
uator into twenty-four equal parts. 



392 MYSTERIES OF TIME AND SPACE. 

catalogue. By an elaborate process of examination, during 
which he showed a singular mastery of the laws of averages, 
Struve estimated how many stars of the different orders had 
been missed in different parts of the zone ; and in this way 
he increased the number of stars virtually available for his 
purpose from 31,085 to 52,199, a number fairly comparable, 
it will be noticed, with the 70,000 stars counted by Sir John 
Herschel and the 90,000 stars counted by his father. 

Struve conceived the zone of stars to be divided into 
twenty-four equal parts by hour-circles, that is, by division- 
lines square to the equator, or to the length of the zone. 
Then, as in his former researches, he compared together the 
numbers of stars in these twenty-four equal divisions. He 
again found that the divisions crossed by the Milky Way 
and those lying near to the Milky Way, are richer in stars 
than the rest. The evidence was of the most decisive 
nature, and Struve rendered it more convincing by the way 
in which he repeated his tests. Thus, whether the twenty- 
four divisions were separately considered, or whether he 
took them in opposite pairs, or whether he took them in 
pairs lying at equal distances on either side of the points 
where the Milky Way crosses the equator, or whether, lastly, 
he compared what may be called the Milky Way divisions 
with the remainder : in all cases he found the evidence de- 
cisively in favour of a condensation of stars, of the leading 
orders of magnitude, towards the galactic zone. For the 
sake of brevity, I quote only the last of the four above-men- 
tioned tests. The Milky Way crosses the equator near the 
hour-divisions 6 and 18 ; more exactly, the main line of the 
Milky Way crosses the equator at the divisions marked 6h. 
40m., and 18I1. 40m. ; so that the divisions corresponding to 
the hours 5, 6, 7, 8, on one side, and 17, 18, 19, 20 on the 
other, are those which may be called the galactic divisions. 
Now, the first set of four divisions contained 13,593 stars, 
the other set 10,657 stars, giving an average of 12,125 stars 
for each set of four divisions, and 3,031 stars for each divi- 
sion. The remaining sixteen divisions contained 27,946 



A SURVEY OF THE NORTHERN HEAVENS. 393 

stars, or an average of 1,747 stars for each. There can be 
no question, then, that a tendency to aggregation along the 
galactic zone exists among the stars of the first nine orders 
of magnitude. 

Struve extended his inquiry, so as to include the con- 
sideration of the distribution of the 52,000 stars in space. 
This he did by conceiving his zone of stars to be converted 
into a flat disc in the plane of the equator, and the stars of 
the several orders spread uniformly as respects radial direc- 
tions — that is, in directions tending from the sun, the 
imagined centre of the system of stars. This part of Struve's 
work seems open to serious objection. The general lesson 
deduced from the former part of the work was good, so far 
as it went ; but, in the use now made of the results, Struve 
certainly pushed them farther than was permissible. What 
he had learned was, that certain divisions of the heavens, 30 
degrees long and 15 degrees wide, were severally more or 
less rich in stars. It seems sufficiently hazardous to lose 
consideration of the length of these divisions, and to sup- 
pose that their several degrees of richness adequately re- 
present the richness of the heavens along their medial line. 
But to proceed farther, and to assume that outwards from 
the eye towards that medial line there was a general uni- 
formity of distribution, was perfectly incompatible with the 
lesson taught by the varying richness of the several original 
divisions. That lesson clearly was, that any assumption of 
uniformity is inadmissible until evidence has been obtained 
which renders it to some degree probable. Struve had not 
a particle of evidence in favour of the assumption he made. 
That assumption amounted, in fact, to the hypothesis that 
there is a general approach to uniformity of magnitude among 
the stars — so that Struve, who had clearly disproved the old 
hypothesis of a general uniformity of distribution, was now 
adopting an hypothesis at least as little likely to approximate 
to the truth. 

Let me now be permitted to present my own researches 
directed to the solution of the same problem. My inquiries 



394 MYSTERIES OF TIME AND SPACE. 

have been in a sense parallel to those made by Struve ; but 
they have been conducted on a. different plan. It will have 
been noticed that Struve dealt throughout with numerical 
statistics, and that he discussed these according to the law 
of averages, regarding minor details as probably unimportant. 
I have followed always the method of equal surface (iso- 
graphic) charting, whereby numerical statistics are in effect 
presented to the eye ; the method having this further ad- 
vantage, that it exhibits (or may be employed to exhibit) 
minor details as well as general relations. 

The value of this method has been impressed upon me 
during a long, and, I believe, quite exceptional course of 
star- charting. In 1866, I drew my Gnomonic Chart, show- 
ing all the stars down to the fifth magnitude inclusive. In 
drawing the maps of my 'Constellation Seasons,' I had 
occasion to picture twelve several times the relations among 
the stars visible in England, down to the fourth magnitude 
inclusive. And afterwards the same series of maps, but 
showing only the stars down to the third magnitude, was 
constructed for my ' Half Hours with the Stars.' It was the 
recognition of peculiarities of arrangement even among the 
relatively small number of stars included in these several 
series, that led to the enunciation of my earliest ideas re- 
specting the laws according to which the lucid stars are 
distributed. 

Then followed the construction of my large Star Atlas, 
in which stars down to the sixth magnitude are included ; in 
other words, about 5,900 stars instead of the 1,500 of the 
Gnomonic Atlas. There was, also, this peculiarity about 
the large atlas, to render the progress of the work more 
instructive than it would otherwise have been : for the first 
time since star-charting began, the stars visible to the naked 
eye were all pictured without appreciable distortion and on 
a systematic plan, bringing into recognition the actually 
existent but hitherto unsuspected laws according to which 
those stars are distributed. It was natural that I should 
take the earliest opportunity to make an actually isographic 



A SURVEY OF THE NORTHERN HEAVENS. 395 

chart of these stars — that is, that I should present these stars 
in a chart in which equal spaces on the globe should be 
represented by equal spaces on the chart. The results of 
this work, and of the application of a method of research, 
which I called the ' balance and scissors ' method, were de- 
scribed in a paper which appeared in ' Fraser's Magazine ' 
several years ago. 1 

But I have long been anxious to apply this method to a 
far larger number of stars than the 5,900 shown in my atlas. 
Large as this number may appear, yet by comparison with 
the numbers dealt with by Struve, and still more with the 
numbers counted in the Herschelian gauges, it appears in- 
significant. And although the conclusions which I had 
actually educed from the distribution of these stars are not 
in reality open to question — since I have been able to show 
that the laws of probability will not permit us to regard the 
observed relations as accidental, yet they may not be judged 
by some so convincingly established, as relations founded 
on an overwhelming array of instances. 

Accordingly, I had for some time desired to chart on a 
single isographic projection, all the stars — 324,000 in num- 
ber — included in Argelander's splendid series of charts of 
the northern heavens. These charts show all the stars of 
the first nine orders and of the order intermediate to the 
ninth and tenth over the whole of the northern hemisphere 
and within two degrees south of the equator. Each chart 
forms a sheet 20 by 30 inches in extent ; and my purpose 
was to combine all these charts into one circular chart on a 
single sheet. 

The first point to be attended to was, that the maps 
should only be commenced when it was possible to carry on 
the work without interruption to its close. To have drawn 

1 These results were first published in a paper read before the 
British Association, during their meeting at Liverpool in 1873. The 
isographic chart illustrates the second edition of my Other Worlds > 
where also the results I deduce from the chart are described and 
analysed. 



■ 



396 MYSTERIES OF TIME AND SPACE. 

a portion of the chart, and then to have left it even for a 
few days, while the work was in progress, was to risk a 
change in the style of mapping, which would have rendered 
the map almost valueless so far as my purpose was concerned. 
But to secure time for the charting of 324,000 stars — 
each star to be carefully copied in from a large original — was 
not an altogether easy matter. On a moderate computation, 
a minute would, on the average, be required for marking in 
ten stars : and this gave 540 hours for the star-charting 
alone ; that is, at the rate of 60 working hours per week, 

9 weeks would be required for the work. It was with diffi- 
culty I could secure 7 clear weeks. During this interval, 
however, I succeeded in completing the chart. 

Before the charting was commenced, the projection was 
laid down in pencil, to every degree, that is, there were 360 
radial lines and 92 concentric circles. Near the centre 
some of the radial lines were omitted. In all, the pencilling 
divided the chart into 26,400 spaces, corresponding to the 
spaces in Argelander's charts. Thus the average number 
of stars in each space was but eighteen ; and it was easy 
to make each space a sufficiently accurate miniature of the 
corresponding space in the large original. When all the 
26,400 spaces were thus filled in, there appeared at a single 
view all the stars which we should see in the northern 
heavens if our powers of vision were increased a hundred- 
fold. Twice as many stars as both the Herschels counted, 

10 times as many as Struve discussed in his ' Etudes,' and 
about 130 times as many as can be seen with the naked eye 
over the whole of the vault of heaven on the darkest and 
clearest night, were now for the first time (thanks to the 
seven years' labour of Argelander) presented to the eye at 
a single view ; and it remained only to study the relations 
which could be recognised among these thirty myriads of 
stars. 

These relations are full, I conceive, of significance. 
In the first place, the aggregation of stars along the 
galactic zone, which Struve had inferred from numerical 



A SURVEY OF THE NORTHERN HEAVENS. 397 

statistics, was amply confirmed. But more than this. I had 
long since expressed my belief that it is not the galactic 
zone, but the galaxy itself, towards which the stars are 
aggregated. The distinction is of importance. When astro- 
nomers speak of the condensation of stars along the galactic 
zone, they imply that there really is a distinct zone of the 
heavens formed by the Milky Way, and that the stars are 
spread with a gradually increasing degree of richness the 
nearer that zone is approached. Now, it has been known 
for many years that so far as the visible Milky Way is con- 
cerned, no such zone exists. Across the widest part of the 
Milky Way there is a broad gap completely marring the 
completeness of the so-called zone. Then, in another region, 
the Milky Way consists of a multitude of interlacing branches, 
intermixed with lakes and islands of light and darkness, form- 
ing a galactic region complex beyond expression. These 
features belong to the southern half of the Milky Way ; and 
we owe our knowledge respecting them to those researches 
of Sir John Herschel which were in progress when Struve 
wrote his ' Etudes.' But even when speaking of the northern 
portion Struve had been constrained to admit that a fresh 
survey was needed, before the Milky Way could be regarded 
as a zone, simply divided into two, along one part of its 
extent. And, accordingly, we find that when carefully 
studied, the northern half of the Milky Way presents a com- 
plexity of structure which the accepted description and 
pictures are far from conveying. Wollaston has referred to 
branchlike projections both on the northern and southern 
side of the zone, in the constellation Perseus, and also in 
the divided portion of the Milky Way from the Swan to the 
equator. Sir John Herschel described how a branch extends 
from Cepheus towards the North Pole ; and another from 
Perseus towards the Pleiades ; neither of these branches 
appearing in Wollaston's description. A single dark space 
is seen in the constellation of the Swan by some observers ; 
but others recognise two dark spaces. Some have regarded 
the second branch of the Milky Way from the Swan as con- 



398 MYSTERIES OE TIME AND SPACE. 

tinuous ; but Sir John Herschel has pointed out that the 
branch does not extend beyond the equator southwards. In 
fine, no two pictures or descriptions of the Northern Milky- 
Way agree even as respects main features, and far less in 
minor details. The cause of this is undoubtedly the real 
complexity of the galaxy, a complexity well described by the 
late Professor Nichol, in the following noteworthy passage : 
1 It is, indeed, only to the most careless glance, or when 
viewed through an atmosphere of imperfect transparency, 
that the Milky Way seems a continuous zone. Let the 
naked eye rest thoughtfully on any part of it ; and, if cir- 
cumstances be favourable, it will stand out rather as an 
accumulation of patches and streams of light of every con- 
ceivable variety of form and brightness ; now side by side, 
now heaped on each other, again spanning across dark 
spaces, intertwining and forming a most curious and complex 
network ; and at other times darting off into the neighbour- 
ing skies in branches of capricious length and shape which 
gradually thin away and disappear. 

Now, in my chart all the chief features of the Milky Way 
are actually shown by the mere marking in of the stars be- 
longing to Argelander's chart. In other words, these stars so 
congregate on the galaxy as to form a picture of it. I had 
thought of marking, on some photographs of the chart, the 
outline of the Milky Way; but this was unnecessary, simply 
because the stars in the chart showed the Milky Way pre- 
cisely as in a stippled engraving. Wollaston's three project- 
ing branches in Perseus, and Sir John HerscheFs branch 
towards the Pleiades, are plainly shown, so also the projec- 
tion from Cepheus towards the Pole, the. great rich region 
in Cygnus ; in fine, all the features commonly described 
and pictured. Not, however, with the uniform tint and the 
well-defined outlines which are shown in maps ; but (pre- 
cisely as Professor Nichol described the Milky Way) with a 
wonderful complexity of interior structure, and the edges 
thinning away and gradually disappearing. 

When we remember that Sir William Herschel during 



A SURVEY OF THE NORTHER N HEAVENS. 399 

his sweeps (as distinguished from his gauges) established the 
general principle, that where the Milky Way is brightest to 
the eye there the stars shown in his powerful telescope were 
most numerous, we recognise the significance of these cir- 
cumstances. We see that since, where the Milky Way is 
brightest, stars of all orders down to the faintest Herschel 
could see, as well as stars of the first nine or ten orders of 
magnitude, are most numerous, it follows that richness in 
stars of the last-named orders is a phenomenon associated 
with richness in all orders of stars down to the faintest seen 
in the 20-feet reflector. This can only be interpreted in one 
way. Let us take a parallel instance. Conceive myriads 
of birds visible over the sky, their real size and nature un- 
known, but two orders of apparent size recognisable. If 
nothing further were known, the inference that all the birds 
were alike in size, but that those which seemed smallest 
were flying at a higher level than the other set, would be as 
probable as the inference that two kinds of birds were flying 
together at one level (of course, all questions of the ways of 
birds is here set aside, for the sake of our illustration). But 
if there were certain parts of the heavens where the birds 
were much more densely crowded than elsewhere, and if it 
was noticed that in these regions without exception birds ot 
both sets were more numerous than elsewhere, no other con- 
clusion could be arrived at than that the two sets were inter- 
mingled, and therefore really unlike in size." For to suppose 
otherwise would be to imagine that two sets of birds flying 
at very different levels, so arranged themselves in their re- 
spective strata that whenever the line of sight from the 
observer on earth passed through a dense cluster belonging 
to one set, it also passed through a dense cluster, much 
more distant, belonging to the other set. This, of course, 
would be too unlikely for belief, more especially if the richly 
clustered regions were very numerous and complex. Pre- 
cisely the same reasoning applies to the observed arrange- 
ment of stars. We cannot suppose that clustering aggrega- 
gations of stars which are relatively near to us, lie always in 



4 oo MYSTERIES OF TIME AND SPACE. 

the same direction as corresponding clusters of stars which 
seem faint through excessive distance. We must, therefore, 
conclude that these seemingly associated groups of faint and 
(relatively) bright l stars, are really intermingled ; in other 
words, that the fainter stars in these groups oive their faintness 
to real relative minuteness. 

Another circumstance is to be noticed in the chart. 
The projections of the Milky Way can be traced farther 
from the main stream than when the galaxy is studied with 
the unaided eye. A clearer idea is thus afforded of the real 
nature of this marvellously complex system of stars. The 
idea which I advocated some time since, that the Milky 
Way is a spiral, is confirmed ; but the spiral can no longer 
be regarded as consisting of one or two whorls lying nearly 
in the same plane. It appears rather as a highly complex 
kind of helix, the spirals of which are not merely numerous, 
but themselves formed of numerous convolutions. 

It is noteworthy, though I am not at present prepared to 
insist on the circumstance, that these branching extensions 
of the Milky Way, when thus traced beyond the limits recog- 
nisable by ordinary vision, seem to unite with the branches 
of the great nebular regions, It is well known that these 
nebular regions lie apart from the Milky Way. In the 
' Proceedings of the Astronomical Society for 1868-69, there 
will be found three engravings, in each of which I have 
given a complete view of these nebular regions, as seen in 
different aspects. The outlying branches are very marked 
in character, and certainly agree in position in a v. ry singular 
manner with regions towards which the branches of the 
Milky Way seem to point. It would almost seem as though 
those mysterious nebular regions are merely distant exten- 
sions of the projecting branches of the galaxy. 2 

1 The faintest stars in Argelander's chart bear about the ^same 
relation to the faintest shown in Herschel's 20-feet reflector that a star of 
the first magnitude bears to the faintest which the naked eye can detect. 

2 This is a matter for further inquiry. I propose to apply a method 
of research to the subject which can hardly fail to educe the true 



A SURVEY OF THE NORTHERN HEAVENS. 401 

One of the features I will touch upon very briefly, 
partly because this paper has already exceeded the limits I 
had proposed to myself when I commenced it, and partly 
because, without illustrations, the real nature of the pecu- 
liarity could hardly be adequately exhibited. I refer to the 
arrangements of the stars in streams and sprays. Here I do 
not speak of star-streams, such as those recognised by the 
ancients in the constellations Aquarius, Hydra, and Eri- 
danus, but to minute wisps (as it were) of stars, whose figure 
seems associated in a significant manner with the regions in 
which they appear. In some parts of the charts this arrange- 
ment is so peculiar and complicated as to give the portion of 
the heavens represented an appearance resembling the twig- 
work of a large tree, as seen projected against a winter-sky. 

But the teachings of the chart are not yet exhausted. 
Peculiarities of structure present themselves, which suggest 
unexpected relations among the orbs which people space. 
The nature of some of these peculiarities can only be under- 
stood by a reference to the chart itself; and respecting 
others, I forbear to speak at present, in order that at another 
time I may discuss them more fully than space will here 
permit. Perhaps the most important result of the construc- 
tion of the chart is the information afforded as to the regions 
of the heavens which are most likely to reward the star- 
gauger. The chart shows what regions of the heavens 
should be selected for this kind of survey. I believe that 
telescopists cannot more effectually advance our knowledge 
of the structure of the heavens than by joining in this much- 
needed work. 

meaning of the observed relations. It is not impossible that in a short 
time I may be able to publish the result of my inquiries. 



i) d 



402 MYSTERIES OF TIME AND SPACE. 



STAR UNTO STAR. 

When nearly twenty years ago, Drs. Huggins and Miller 
published the first results obtained from the spectroscopic 
study of stars, few could have supposed that a line of re- 
search so difficult and delicate would lead to the bold and 
startling views of the star-depths which now seem opening 
out before us. Still less would it have been thought that 
the method of research would be so modified that the ob- 
servations belonging to it could be pursued without the 
direct personal study of the stellar spectra which have been 
found so difficult, and even (where exact researches were in 
question) so painful. In 1864 the observer who wished to 
determine whether a special substance existed in the vapor- 
ous atmosphere of a star, had to compare the spectrum of 
the star directly with the spectrum of the substance. In 
other words he had first to turn his telescope upon the star 
with such precision that the image of the star should fall on 
the fine slit of the spectroscope (and be kept there by clock 
motion driving the telescope, throughout the operation), 
and the light of the star being then sifted out by the action 
of the prisms in the spectroscope, so as to form a rainbow- 
tinted streak or spectrum crossed by dark lines where certain 
tints are missing (on account of special absorptive action in 
the vaporous atmosphere of the star), the observer had to 
bring into the same field of view, and into precisely corre- 
sponding position, by the action of the same spectroscope, 
the bright line spectrum of whatever substance he wished 
to deal with. If the bright lines forming the spectrum of 
magnesium, or sodium, or calcium, or the like, were found 



STAR UNTO STAR. 403 

to correspond exactly with dark lines or missing tints in the 
spectrum of the star, then the observer would know that 
the particular substance giving those bright lines (or, more 
correctly, shining with those tints) existed in the atmosphere 
of the star. But he might very well be in doubt as to the 
precise accuracy of the coincidences (on which everything de- 
pends), or he might not be able to perceive clearly, yet might 
suspect the existence of one or other of the dark lines 
necessary to complete the evidence. To make sure he must 
cause the electric spark producing the spectrum of the sub- 
stance he is dealing with to flash again and again out of the 
darkness, wearying the eye by the constant alternation of 
darkness with bright light. Not a few minutes, but many 
hours, on even several observing nights, would be required 
for each observation of the sort ; and later, some other 
observer, with different visual powers, or with instruments 
of greater or less precision, might throw doubt on the accuracy 
of the observation, and the whole work might have to be 
repeated. 

Now, all this is changed. A photographic record of the 
spectrum is taken (hitherto only of the blue, violet, and 
ultra-violet part, but before long the whole visible spectrum, 
and parts invisible beyond the red and violet, will doubtless 
be photographed), and either at the same time or under 
precisely the same optical conditions, a photograph of the 
sun's spectrum (not taken directly from the • sun, but either 
from the twilight sky or from a planet like Venus which 
reflects pure sunlight), and then the known dark lines in the 
solar spectrum can be compared directly with the dark lines 
in the spectrum of the star. If doubt be afterwards thrown 
on the result, the slips with the recorded photographic spectra 
are always available for comparison. And thus star after 
star can be added to the list of those whose light-record of 
their vaporous structure has been obtained. Fainter and 
fainter stars can be dealt with as the delicacy of sensitive 
plates is increased, or as the accuracy of the clock-driving of 
telescopes is increased, until the photographic plate may be 



HBHHH 



404 MYSTERIES OF TIME AND SPACE. 

exposed during the whole of any clear night to receive the 
light impressions from a star. Already Dr. Draper has 
obtained records of the spectra of stars of the tenth mag- 
nitude — that is, far beyond the range of ordinary vision — 
though as yet such records of faint stars have not been 
available for the kind of research we are considering. In 
fact they have only been received accidentally, so to speak, 
when search was being made for something entirely dif- 
ferent. 

We are not, however, here concerned to consider at any 
length the methods employed. It is interesting, and will 
appear more so as we proceed, to note how widely the re- 
search we are considering is likely to be extended in the 
future. But at present we propose chiefly to discuss the 
most remarkable result which has rewarded the method of 
spectroscopic inquiry into the stars, whether by ordinary 
vision or by the use of photographic appliances. 

The result to which we refer is the marshalling of the 
stars into orders, different in colour, which spectroscopic 
analysis shows to be due to difference in present physical 
constitution, which again analogical reasoning shows to be 
due to difference in age. 

Take first the bluish- white stars of which Sirius, Vega, 
Altair, and others are typical. 

In the first place, we note that the only star of this order 
whose distance has been even roughly determined (Alpha 
Centauri in the southern hemisphere is a yellowish-white 
star) is demonstrably a much larger orb than our own sun, 
if the quantity of light which a sun emits is any indication of 
size. Sirius is so remote that the motion of the earth in her 
vast orbit, 185 million miles in diameter, scarcely at all affects 
the apparent position of that brilliant star. Very exact and 
careful study of the star indicates apparent motion due to 
the earth's real motion in a tiny ellipse, the larger axis of 
which is roughly about the 4,000th part of the moon's ap- 
parent diameter — the nature of the observation being such 
that this larger axis may be as much as the 3,000th or as 



STAR UNTO STAR. 405 

little as the 5,000th part of the moon's apparent diameter, 
or even lie outside those limits. Taking the mean of the 
best measurements, a distance is inferred so great that our 
sun's light, were he placed at that distance, would be re- 
duced to about the 50th part of the apparent lustre of a 
first-magnitude star, or, roughly, to about the 200th part of 
the lustre of Sirius. Hence it would follow that if an average 
square mile of the surface of Sirius emits as much light as 
an average square mile of the sun's surface, the surface of 
Sirius must be 200 times as large as the surface of our sun. 
If so, the diameter of Sirius would be about 14 times the 
diameter of the sun (for 14 times 14 are 196), and his volume 
about 2,800 times, or in round numbers 3,000 times the 
volume of the sun. We can hardly suppose that his volume, 
or probably his mass, is less than a thousand times larger 
than the sun's. 

Of other stars of the bluish-white order we know less, 
with precision, but we do know so much as this, that all the 
brighter ones must be, and therefore even the fainter ones 
probably are, very much larger than the sun. For though 
the actual distance of Vega and Altair, for example, cannot 
be determined, it is because they are so far away that at- 
tempts at measurement fail. If either of them were as near 
as Sirius, its distance would be as readily determinable. 
But the measures which, applied to Sirius, give a recognis- 
able result, fail utterly when applied to Vega and Altair. It- 
is true, results are published in our books of astronomy 
which if accepted would indicate a measured distance in 
the case of Vega, but it is utterly untrustworthy. Vega and 
Altair lie beyond the range of the best methods of measure- 
ment yet invented. But noting that their lustre still exceeds 
many times that which the sun would have if removed to 
the distance of Sirius, we infer safely that the lustre of those 
two bluish-white stars exceeds in yet greater degree that 
which our sun would have if removed to their distance : in 
what precise degree we cannot determine, but we may con- 
fidently say that these stars are very much larger than our 



H 



406 MYSTERIES OF TIME AND SPACE, 

own sun. The same argument applies to all the brighter 
stars of the bluish-white kind. And having thus inferred 
that so many stars of this colour exceed our sun in size, it 
is a highly probable inference that all do (the fainter being 
simply much farther away), if it shall appear that all 
the stars of this kind possess certain physical characteristics 
which stars of other colours do not. For if it is a fair inference 
that because all bluish-white stars yet examined possess 
such characteristics, so will others of the same colour which 
may hereafter be examined ; and, again, that because no 
other stars have yet been found to possess these character- 
istics but stars of a bluish-white colour, therefore others 
which may hereafter be found to possess them will also be 
of this colour, it is clearly as fair an inference to assume that 
the great size characterising all the stars of this kind yet 
tested or testable in this respect is a characteristic also of 
the class. 

Now it appears from direct spectroscopic study of these 
stars, as well as from their spectra, that they differ in physical 
structure in marked manner and degree from our sun. The 
lines which indicate the presence of relatively cool hydrogen 
■ — hydrogen exerting an absorptive action on the light from 
the central glowing mass — in these stars are always much 
stronger and broader than in the spectrum of our sun. I 
do not dwell here on a question which has arisen as to a 
certain line which appears to be common both to calcium 
and to hydrogen, and has therefore given rise to certain 
discussions (running, in my opinion, far in advance of the 
evidence) as to the identity of some element common to 
both calcium and hydrogen, which of course, according to 
that view, would neither of them be elements. I prefer now 
to consider only the broad lines of distinction between the 
various orders of stars, and not to discuss minutice which 
may hereafter very probably be shown to be altogether 
without significance. 1 

1 Just as Professor Young, by using spectroscopes of great disper- 
sive power and showing lines to be diverse which with inferior instru- 



STAR UNTO STAR. 



407 



Now the great breadth and strength of the hydrogen lines 
in these monstrous suns (suns exceeding our sun much as 
oar sun exceeds Jupiter and Saturn, and as these planets 
exceed our earth, Venus, and Mars) may be taken safely 
enough to indicate the existence of much deeper and denser 
atmospheres of relatively cool hydrogen around those suns 
than exist around our own. Yet the intense brightness and 
whiteness of those suns serve to show that such deep en- 
velopes of relatively cool hydrogen are by no means due 
to the longer continuance of a process of cooling. On the 
contrary, it seems clear that it is the greater intensity of the 
radiation of those parts of the stars' light which form the 
continuous background of the spectrum, and not the greater 
intensity of the absorptive action of the hydrogen, which 
really occasions those lines to appear broad and dark. The 
hydrogen itself, which, owing to the great lightness of this 
element under the same conditions of temperature and 
pressure, extends high above the other gaseous envelopes, 
forming the outer parts of these intensely bright white stars, 
is no doubt itself intensely hot. Most probably it is far 
hotter than those hydrogen layers which cause the finer 
absorptive lines of hydrogen in the spectrum of our own sun 
and his fellows. But so much more intense is the light 
radiated from the glowing mass within (mostly from glowing 
gas at great pressure, I think) that the absorptive lines of 
hydrogen appear by contrast very broad and very strong. 

On this view we may fairly assume that the darkness of 
the hydrogen lines is a characteristic of stars at a much 
higher temperature than our sun and suns of the same class. 
And finding this characteristic associated with some stars 
which are certainly of enormous size, and with other stars 
which may be thus exceptionally large, we are led to infer 
that this association is not accidental — that all stars having 



merits had seemed identical, has entirely destroyed the imagined validity 
of evidence on which certain very bold assumptions as to the elementary 
constitution of matter had been based. 



4 o8 MYSTERIES OF TIME AND SPACE. 

these very strong hydrogen lines are very much larger than 
own own sun. 

Whether we can accept this inference or not will depend 
very much on whether we can regard the youth of a sun as 
in any way correlated with the sun's size. The reasoning 
which I have applied to planets — the justice of which 
reasoning seems confirmed by the accordance of the results 
to which it leads with observed facts — maybe applied also to 
the stars. I have shown that if two planets of different size are 
at any given epoch in the same stage of planetary life — that 
is, at the same temperature — the smaller will presently pass 
into a more advanced stage than the larger will have attained 
to, because it will part with its heat at a relatively greater 
rate. Supposing, for instance, the diameter of the larger 
planet is twice as great as that of the smaller, and therefore 
the surface four times as great, and the volume (or mass, if 
the planets are of nearly the same density) eight times as 
great as that of the other, it is evident that as the quantity 
of heat is proportional to the quantity of matter, or eight 
times as great in the larger planet when the two are at the 
same temperature, while the rate of emission, being propor- 
tional to the surface, is but four times as great, the supply 
of heat in the larger will last twice as long as the smaller 
supply of heat in the smaller planet. Now, as I have said, 
this reasoning applies equally to the stars ; and if we could 
only be assured that at any given time two stars of unequal 
size were in exactly the same stage of stellar life, we should 
be sure that at any much later stage the smaller star would 
be much more advanced in stellar life than the larger. 

The difficulty arises here, however, that we have no means 
of proving, but on the contrary strong reason for doubting, 
whether the stars of our galaxy began their ^existence of 
stars at any common time. When we see the various orders 
of nebulous masses within the galaxy, and note how very 
different seem to be the states of these nebulae as to condi- 
tion, while the very existence of true nebulae (many of which 
we may regard as unformed suns or star-clusters) indicates 



STAR UNTO STAR. 



409 



the great diversities of age existing among the occupants of 
stellar space, we perceive how very unsafe it would be to 
assume that the stars, simply because they are stars, began 
their existence as such all at the same or nearly the same 
time. The contrary is not only far more probable, but to 
all intents and purposes certain. 

All we can safely assume is, that the greater size and 
mass of a star indicates the much longer continuance of all 
the stages of its career, past and to come— that it has been 
much longer in passing through the inchoate stage, and 
through its first stages as a formed sun* and that it will be 
very much longer in passing through all those stages which 
it has still to go through, than our own sun or other suns of 
the same class. Looking at such a sun as Sirius, for exam- 
ple, we may believe that at the beginning of its present 
stage of existence as a bluish-white sun, our sun and Sirius 
may have both been bluish-white, but that our sun, being 
very much smaller, has passed onwards into the stage when 
a star shines with yellowish-white lustre, and will perhaps 
pass onwards to the later stages of which we have yet to 
speak, while Sirius and Vega are still shining as bluish- white 
stars. But we cannot assume that any small bluish-white 
star which gives (as many small stars do) the same sort of 
spectrum as Sirius, is in reality an enormously large sun, 
another Sirius in fact, shining with the same sort of light 
because, beginning its existence at about the same epoch, it 
has taken a much longer time than our sun to reach the 
same stage of sun life. It may be that a bluish-white star, 
with strong hydrogen lines in its spectrum, is no larger than 
our sun, or is even smaller than he is ; but having come 
into existence as a sun much later, has not reached the 
same stage of development. 

It is important that we should not here fall into an error 
of the same sort as that which vitiated the earlier reasonings 
of Sir William Herschel respecting the stellar distances. He 
regarded the' brightness of a star as fairly indicating its dis- 
tance, assuming all stars to be of the same general order • 



4io MYSTERIES OF TIME AND SPACE. 

later we see a tendency on his part to fall into the opposite 
extreme., and regard brightness as rather indicating the real 
size of a star than proximity. Neither inference can in 
point of fact be relied upon ; some faint stars are large ones 
very far off ; others are really small stars not farther off, or 
even nearer, than their fellows. 

So it is in the case before us. Some bluish-white suns 
with spectra indicative of stellar youth are no doubt enor- 
mously large orbs, compared with which our sun is little 
more than as a dwarf compared with a giant ; such suns are 
young because they- s are large ; the stages of their lives are 
all very much shorter than the corresponding stages of the 
lives of our sun. But others no doubt of these young suns 
are really young in years as well as in development ; they 
are younger than our sun, not because they require longer 
time-intervals for the various stages of their life, but because 
they began their stellar life later. 

Note in passing that these spectra of the bluish-white stars 
are not all exactly alike. They are distinguished from each 
other by the greater or less breadth and diffuseness of the 
lines of hydrogen, and also by various degrees of strength 
and visibility of the finer lines. It may possibly be that 
hereafter, in such distinctions as these, we may be able to 
recognise evidence as to the size of a star — that, for instance, 
a large star in passing through the first stage of stellar life 
may present characteristics always different in certain re- 
spects from those presented by smaller orbs in passing 
through the same stage. If so, we shall have a new means 
of dealing with the architecture of the heavens ; for, knowing 
something of the real size of a star in this way, we may infer 
its distance from its apparent size, and thus place it correctly 
in position in space, instead of knowing only the direction 
in which it lies, at some distance unknown. 

Pass now to the next order of suns, of which Aldebaran, 
Capella, and our own sun are examples. ' In the spectrum 
of Aldebaran,' says Dr. Huggins, ' the lines of hydrogen are 
reduced to about the proportion they possess in the solar 



STAR UNTO STAR. 



411 



spectrum ; the other lines of the spectrum are no longer 
fine and difficult to see ; we have in full the triple line of 
magnesium.' I have seen the spectrum of Capella as photo- 
graphed by Professor Henry Draper of New York, for com- 
parison with the spectrum of the sun, as received after 
reflexion from the surface of Jupiter. Matters were so 
arranged that the two spectra were of the same strength. 
Now when these photographs were placed side by side (the 
corresponding dark lines being brought into the same direc- 
tion, so that the eye could run along a dark line of Capella 
into the corresponding dark line of the sun, I found it 
almost impossible to recognise the slightest difference be- 
tween the two spectra. Almost every line in the spectrum 
of Capella corresponded with a dark line in the spectrum of 
the sun ; in each case the strength of the lines corresponded 
very closely. Only after a prolonged and close scrutiny 
could I satisfy myself that one or two lines of the solar 
spectrum seemed slightly stronger than the corresponding 
lines in the spectrum of Capella, and in these cases I found 
that these very lines are known to be slightly strengthened 
by absorptive action experienced as they pass through the 
atmosphere of Jupiter. In this case, apart from a slight 
disturbing influence due to this absorptive action, a com- 
parison was made between our sun and Capella, precisely as 
from a world travelling round a sun equidistant from these 
two orbs. The practical identity of the two spectra is the 
best proof yet afforded of the oneness of constitution (with 
infinite variety of distribution) throughout our galaxy. 

Again, however, we find ourselves confronted by a diffi- 
culty akin to that already experienced in dealing with the 
question of the relative dimensions of the bluish- white stars. 
Only that, whereas in their case we could only recognise 
the extreme probability that many stars of that order differ 
largely in size from Sirius and Vega, we have in the case of 
stars of the second order not only probable inference of this 
sort, but proof positive that two at least among the stars of 
the second order differ exceedingly in size from our own sun, 



412 MYSTERIES OF TIME AND SPACE. 

For although we do not know the actual distance of 
either Capella or Aldebaran (I disregard utterly all the 
measurements of Capella's distance which are given in our 
books of astronomy, or rather I regard these as proving 
conclusively that Capella lies utterly beyond the range of 
measurement *), we do know certainly that our sun placed 
at the distance of either of these stars would shine with very 
much less light than either of them. We know that, set 
beside Alpha Centauri, he would shine with about a third 
part of the light of that star. Now, Capella shines with 
almost exactly half the light of Alpha Centauri, and Alde- 
baran with about three-sevenths. Thus our sun set at the 
distance of Alpha Centauri would shine with about two-thirds 
the lustre of Capella, and about seven-ninths the lustre of 
Aldebaran. But each of these stars is at least five times 
farther away than Alpha Centauri, or otherwise the persistent 
efforts made to determine the distance of each must long ere 
this have been rewarded with more success than astronomers 

1 It is singular that any faith should be placed by professional 
astronomers in measurements so manifestly untrustworthy as those 
which have been given in the case of stars like Capella, Polaris, and 
Arcturus. When we remember that the star 61 Cygni, which comes 
next in distance— probably— to Alpha Centauri, was first set by the 
careful measurements of Bessel some three times as far away, and then 
brought by the equally careful and refined measurements of Peters to 
only twice the distance of Alpha Centauri — a correction of twenty 
millions of miles, or more than three years' light journey, we see how 
utterly unreliable must be estimates like those (due to Peters) which 
set Arcturus about eight times, Polaris about fourteen times, and 
Capella about twenty-one times as far away as Alpha Centauri. The 
error in the determination of the annual displacement of 61 Cygni 
was fully one-sixth the annual displacement of the nearest star in the 
heavens — Alpha Centauri^-the only star injny opinion whose distance 
has been fairly, though roughly, measured. Of what use, then, to 
give us the annual displacements of the three stars named, when even 
that assigned to Arcturus is only the eighth' of that nearest star's — that 
is, the whole displacement which Peters claimed to have observed in 
the case of that star — only three-fourths of the discrepancy between his 
result and Bessel's in the case of another star ? 



STAR UNTO STAR. 



413 



have hitherto attained in this direction. Thus each would 
look at least twenty-five times as bright as it actually does 
if removed from its present distance to that of Alpha Cen- 
tauri. Therefore Capella may fairly be assumed to give 
about forty times (roughly) as much light as our sun at the 
same distance, and Aldebaran at least thirty times as much. 
But in the case of two stars whose spectra are very similar 
to the spectrum of our sun, we may fairly assume that (on 
the average) each square mile of surface gives out about the 
same quantity of light as (on the average) each square mile 
of the surface of the sun. It would follow on this assump- 
tion, which is not a very bold one, that the surface of 
Capella is about forty times as large as the surface of the 
sun, and the surface of Aldebaran about thirty times as 
large — say, for convenience of calculation, thirty-six instead 
of forty in the former case, and twenty-five instead of thirty 
in the latter. Then it would follow that the diameter of 
Capella is six times as great, that of Aldebaran five times as 
great, as the diameter of our sun. Hence the volume of 
Capella would be (216 times) more than 200 times, and the 
volume of Aldebaran (125 times) more than 100 times our 
sun's. Of course the calculation is very rough, and a great deal 
is assumed. Albeit nearly all the assumptions have been 
such as rather to diminish than to increase our estimate of 
the size of these seemingly giant suns of our sun's own order. 
It is certain Capella and Aldebaran are at least five times 
farther away than the sun — they may be very much farther 
away even than that. There is no room for doubt about 
the photometric measurements by which the relative bright- 
ness of the sun, Capella, and Aldebaran, at the same distance, 
has been determined. It maybe, perhaps, doubtful whether 
the intrinsic brightness of the surface of our sun is so nearly 
the same as that of the surfaces of Capella and Aldebaran 
as to leave the estimate we have formed appreciably un- 
affected by whatever correction may be due to this cause ; 
but be it noticed that we have already made a correction, 
since we have reduced the estimate of Capella's surface 



4*4 MYSTERIES OF TIME AND SPACE. 

from forty to thirty-six times, and that of Aldebaran's from 
thirty to twenty-five times, that of the sun's surface. 

Now, if Capella really has a diameter six times greater 
than the sun's, every stage in the cooling of Capella — that 
is, every stage of this star's life — would probably last about 
six times as long as the corresponding stage in the life- 
time of our sun. For the volume being on this assump- 
tion 216 times as great, it would be in that degree that the 
quantity of heat in Capella, at any the same stage of its 
existence, would exceed the quantity of heat in the sun, 
whether we consider actual or potential heat arising from 
the contraction due to gravity. The heat would pass away 
from a surface only 36 times greater, that is, not 216 times 
as fast (which would make the supply last just as long, but 
at one-sixth that rate) ; therefore the supply would last 
about six times as long. In the case of Aldebaran the sup- 
ply for each stage of star-cooling would last about five times 
as long. These numbers are, of course, very far from exact- 
ness ; but they suffice to show that the lifetime of one star 
of a given class or order may exceed very much in duration 
that of another star of the same kind. 

We come next to the stars or suns of the third order, 
whose light, instead of being bluish- white like that of Sirius 
or Vega, or yellowish-white like that of Capella or of our 
sun, is of an orange-yellow tint. The best representative of 
this class of sun is Arcturus, whose spectrum is somewhat like 
that of our own sun, but presents characteristic peculiarities, 
which the late Father Secchi regarded as corresponding to 
what we might expect in a sun like ours at a time when a 
great number of spots were present on its surface. If we 
adopt this opinion, we should regard Arcturus as a perma- 
nently spotted sun. Dr. Fluggins merely remarks of Arc- 
turus that it is a star of another order, which includes the 
solar type, but the star seems to be removed farther than 
the sun is in the order of change from the typical form as 
we meet it in Vega and Sirius. Here the typical lines are 
no longer present as a strong group, The line which has 



STAR UNTO STAR, 



4*5 



been regarded as belonging to both calcium and hydrogen 
is stronger, relatively, than in the solar spectrum. The 
spectrum of this star is crowded with fine lines, and in the 
visible part resembles the solar spectrum, but in the ultra- 
violet part, which hitherto alone photography has recorded, 
the lines are more intense than in the solar spectrum, and 
are differently grouped. 

The inference from the observed peculiarities of the 
spectrum of the star Arcturus is that this is a sun further 
advanced in sun-life than our own. 

Now, here again the question as to size is answered in a 
way suggesting that there is no present correlation between 
the size of a star and its age or state of development. So 
far as size is concerned, Arcturus, if it had begun its exist- 
ence as a sun at the same time as our own sun, should have 
been much less advanced than he is. For Arcturus is half 
as bright again as Capella, yet lies at least as far away as 
that distance which we have assigned as the least possible 
distance for Capella. Therefore all that we have said 
about Capella and Aldebaran applies with increased force to 
Arcturus. His surface is probably at least sixty or seventy 
times as large as Capella's, even if we assume that the 
intrinsic brightness of the surface of this older star is equal 
to that of our sun's surface ; but it is probably less, in which 
case to account for the great amount of light emitted by 
Arcturus we must assume the surface to be greater in pro- 
portion as its intrinsic brilliancy is less. Even with a surface 
only sixty- four times as great as the sun's. Arcturus would 
have a diameter exceeding his eight times, and a volume 
exceeding his nearly five hundred times. Arcturus would 
therefore be a sun marvellously surpassing our own in 
volume, and presumably in mass also. We may infer, 
reasonably enough, that the family of worlds over which this 
mighty orb bears sway surpasses in like degree in dignity 
and importance that ruled over by our own sun. 

In passing, let it be noticed that all these considerations 
as to the great size of many, if not most, of the stars of the 



4 i6 MYSTERIES OF TIME AND SPACE. 

first orde (the bluish- white suns), of some at any rate of the 
stars of the second order (the yellowish-white suns), and of 
one at least of the stars of the third order (the orange- 
yellow suns) are enormously, one may say overwhelmingly, 
strengthened, if we accept Dr. Siemens' view of the ex- 
haustion of each sun's rays as they do their work in space. 
For in that case all the stars must emit very much more 
light than we have been assuming that they do. In fact, if 
that theory were true, the mere visibility of a star at the 
distance of Sirius would imply that- the sun so seen across 
depths of space exceeding at least a million times the entire 
span of the earth's orbit, was an orb compared with which 
our sun is less than the tiniest meteor compared with the 
mighty mass of our earth. For our own sun, if he does 
anything like the work assigned him by Dr. Siemens, must 
exhaust all his light-giving as well as heat-giving energies 
long before he can extend the news of his existence as a 
sun even to the distance of the nearest star. Yet there in 
the star-depths are ten thousand suns which do much more 
than merely make themselves visible athwart such distances, 
some of them even giving hundreds of times as much light 
as our sun would give if — without any such exhaustion of 
his rays in space — he shone from beyond such distances as 
separate those orbs from us. 

But apart from all such questions as these, there is to 
me something most impressive in the thought of what, as 
thus interpretated by spectrum analysis, the heavens reveal 
to us. Of old it was known that one star differs from 
another in glory — meaning perhaps in brightness only. In 
colour, too, it had been seen that the stars are unlike. But 
who would have ventured to surmise that in real size the 
suns that people space are so unlike? Who could have 
supposed that any instruments men could devise would 
enable us to judge which are the younger, which the older 
stars ? Yet even the most cautious among our astronomi- 
cal physicists, Dr. Huggins, ablest of our spectroscopists, 
accepts this as the only reasonable solution of the observed 



STAR UNTO STAR. 417 

differences in star spectra. ' We cannot resist/ he says, 
' the feeling that in Arcturus ' (and the other stars of that 
class) ' we have to do with a star which has departed farther 
from the condition in which Vega now is than our sun has 
yet done. The question presents itself, Have we before us 
stars of permanently different orders, or have we to do with 
some of the life-changes through which all stars pass ? Does 
the sun's position, somewhere before Arcturus in the order 
of change, indicate also his relative age ? On these points 
we know nothing certainly/ ' If I may give some play to 
the scientific use of the imagination,' he added, addressing 
his audience at the Royal Institution, ' I would ask you to 
imagine an inhabitant from some remote part of the universe, 
seeing for the first time an old man with white hair and 
wrinkled brow, to ask, " Was he born thus ? " The answer 
would be, " No ; in this child, this youth, this man of mature 
age, you see some of the life-changes through which the old 
man has passed." So, giving play to the scientific imagina- 
tion, there may have been a time when a photograph of the 
solar spectrum would have presented the typical lines only 
which are still in Vega. At a subsequent period these would 
have been narrower and more defined, and other lines would 
have made their appearance. And if we allow this scientific 
imagination to project these Royal Institution Friday even- 
ings into the far future, the lecturer, clad it may be in the 
skin of a white bear, may have to describe how the spectrum 
of the then feeble sun has already passed into the class of 
spectra distinguishing those stars which shine with red 
light.' 

It is evident that our great astronomical physicist recog- 
nises no perpetual energy in suns, even in the mightiest. He 
sees them passing downwards along the scale of stellar being, 
gradually parting with more and more of their stored-up 
energies, not recruiting themselves with their own energies 
stored up after doing their full work. And in this, with all 
respect to an eminent practical physicist, he shows himself 
more philosophical as well as more practical. He recognises 

E E 



4 i8 MYSTERIES OF TIME AND SPACE. 

that the same law which affects the small and the short-lived, 
the large and the long-lived must also submit to. Practically 
eternal though to our conceptions the duration of each stage 
of a sun's life may be, each such stage is nevertheless finite, 
even though a sun exceed our own a million times in volume 
or in mass. The heavens present to us a scene of tremendous 
— nay, of inconceivable energy. Suns upon suns, to millions, 
to tens of millions, to hundreds, even to thousands of millions, 
occupy space around us. In every stage of stellar life they are 
at work, illumining, heating, and guiding the systems which 
circle around them. Beyond the limits of the most powerful 
telescope lie thousands of millions more, repeating the same 
story of seemingly infinite energy, seemingly endless dura- 
tion. Yet each one of those orbs, and therefore the sun of 
all, or the universe as we know it, tends to an end — an end 
which may be, however, but the beginning of new forms of 
existence, while the gaseous nebulae, now mere masses of 
vapour, may then have entered on sun-life, to carry on the 
same story, to teach the same lesson, that though each order 
of created things tends onwards to an end, yet to such orders 
we can trace no visible limit — ' End is there none to the 
universe of God ; lo, also, there is no beginning.' 



THE END. 






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